Scott, I'm going to take a whack at this, line by line. First, on your own, you have done a pretty good assessment of the risks and the work that is likely necessary; you seem well-grounded about the prospect of repairs with this car. You also do not seem risk-averse - and it may be a gamble. But it could be a gamble that pays off.
Going into it, the car has been neglected, as evidenced especially by the lack of the engine cover and the low fluid lights. Who treats an S55 that way?
SRS warning: You have a good understanding of what is involved. One hopes the problem is not the main SRS controller (truth be told, I have never seen that particular malfunction reported here). More likely it is the passenger seat sensor, or it could be tied in with the pre-safe warning. The underseat sensor runs about $1K US for the part alone - which is why folks sometimes bypass the sensor (NOT something I recommend). Alternatively, it could be dirty contacts on the plug under the seat, or on the seat belt latch itself.
Pre-safe inactive could indicate a problem with the seat belt system itself (and be tied to the SRS indication) or it could be from the need to reset it with a simple procedure (the latter especially if there are other lights you don't mention). See "Synchronizing ESP" at
http://www.benzworld.org/forums/w220-s-class/1302190-ive-had-2.html#post2216730 and give that a try. If the problem is because of the squib that fires and retracts the seat belt, expect about $1K US for repair, again. Or, the light could have been triggered by a low battery, and the reset procedure will take care of it.
The low coolant message is a bit of a concern. Look first for radiator leaks, especially at the points where hoses connect to the radiator. The lower hose connection is plastic, could crack, and means replacing the radiator. If the radiator is OK, look at the fluid color; is it the right color for MB-approved Zerex G05 fluid? If not, you might research the forum for the several discussions of alternatives - but most of all, you don't want some fluid that may corrode the radiator. And last - how low is the fluid, and if the overflow tank is dry, how long was the engine run in that condition? You could be in for a ring job (check that the exhaust pipes don't show oil). However, if there is fluid in the overflow tank and it is just low, you're OK - and you were going to flush & change it anyway.
Low windshield fluid - Missing headlight washer pop-up? Disconnected tube near the pop-up? Disconnected or broken connection at the washer reservoir? See the last several entries for the windshield washer reservoir under Windshield Wipers at
http://www.benzworld.org/forums/w22...lass-encyclopedia-start-here.html#post5758758 - and note especially the link at
http://www.benzworld.org/forums/w220-s-class/1904297-removing-windshield-washer-reservoir-tips.html -- if the problem is at the washer pump outlet, there are several inexpensive fixes - but you're in Canada. You probably want the heated line to the pump, and the "cheap" fixes won't work for you. Still, not prohibitively expensive, just a PITA to fix.
Tele-Aide visit workshop - the '03 S55 used an analog version of the Tele-Aid equipment. As in the US, Canada took down the analog frequencies that were once used for cellular phone systems, and reassigned them to wi-fi. It is likely your Tele-Aid module is seeking a signal that no longer exists, and that is the root cause. Solutions: Have the dealer code the Tele-Aid out of the system (yes, they can, no matter what some dealers may say) - OR, pay for a digital upgrade and switch to MBRACE, the subscription service that replaced Tele-Aid. The digital upgrade is a bit more than $1K US, but should include 1 year or more of MBRACE service. What do you get? Phone assistance with car problems, ability to have them remotely unlock the car, ability to have the car's location traced by GPS if it is stolen (an optional service), concierge service (option again) and automatic reporting of your location by GPS if your air bags deploy (the reason I have the service). BTW - the tele-aid module is under the rear seat, passenger side. The trunk problem is likely not related to it. See
http://www.benzworld.org/forums/w220-s-class/1834026-w220-antenna-system-key-fob.html for a better understanding.
Trunk won't close - check the audio system. If it works, it is unlikely there has been serious leakage into the electronics bay, where the amp, voice control, audio gateway, phone module, and CDC are located. However, you can check by peeling back the left (driver) side trunk lining and looking for water in the bay. While you're at it, be sure your brake lights and parking lights, work, and also your rear fog light (in back there is one ONLY on the driver side; turn the headlights on, then pull the light switch out fully to turn on fog lights). If water has leaked, corrosion in the contacts can cause problems (which can usually be fixed by cleaning them).
No service history - judging from the car, the reason is it was rarely serviced. Sitting for 2 months without running MAY be the source of some of the warning lights (if the battery was allowed to run down). If the car is cared for, once you bring its condition back up, the mileage isn't of much concern (but you probably won't be able to get a 3rd party warranty).
You will need to investigate the trunk actuator problem more closely; fortunately, there is help. See Power assist trunk (boot) open and close and how to reset + replacement of struts
https://www.youtube.com/playlist?list=PLytZQl-DkR3u2qieg5na5iDEyLWL6qIXa,
http://www.benzworld.org/forums/w220-s-class/2016081-general-w220-information-few-recent-fixes.html) and especially "Refilling the power assist trunk opening reservoir:" Post 21,
http://www.benzworld.org/forums/w220-s-class/1683413-saga-my-new-mistress-s600-3.html#post5640847 - those links are in the Encyclopedia, but I'm posting them here because of your time constraints. It is definitely a problem that needs to be fixed, the sooner the better.
All that said, it is taking a risk. You're buying a neglected car, and you may have to put 5 or 6 thousand dollars (US) into it in the near term. You would still wind up with a car for which you invested less than market value. The question is, do you have the time and the inclination?
I would love to see such a car brought back - the S55 is a great car when cared for (perhaps not the best for ice & snow - a 4matic would be a safer bet). But still, a great car. About 4 years ago I bought a neglected '78 450SL. It had a good engine and transmission, and fairly little rust. I hated to see it in the condition it was in. My $3500 "investment" has cost me $20,000, and at best the car is worth only half that if I were to sell it. But you know what? - It is now a classic in great condition that my wife and I love to drive, top down and windows down, in Florida's beautiful weather. Its value to me equals what I have put into it, despite its low resale. I have enjoyed working on it (except the stuff under the dashboard - those things were back breakers, and I'm getting too old for that). Sometimes, it's not just the money. It is what else you might get from the project - and that is something only you can decide.