#1 - Fog Light Replace, It throws the malfunction for Fog Light, the manual says to take it to the dealer but I am assuming this can be DIY by simply removing the under engine pan to get to the fog light assembly from the rear to replace the bulb?
Yep, and they should be H7 bulbs.
#2 - Coolant Level Warning first thing in the morning, I am going to top off the coolant but want to check which kind/brand is acceptable? I prefer to be able to get it from Autozone (You can judge me later lol) so I can avoid the MB dealer gouging but it will be what it is
Zerex G-05 should be readily available and is compatible.
#3 - Left Turn Not "Sticking" - I'm having to hold this down and my searching has found that the combo switch has to be replaced, in my searching there was mention to make sure to get the correct part number - how can I verify, be sure to get the correct part number? I've already done and saved out my MB Vin Decode. Also I saw warnings not to triggering the SRS error - does disconnecting the battery successfully avoid the trigger of the SRS error on reassembly and startup?
The SRS system is disabled with the ignition off, but if it makes you feel better, pull the negative lead on the battery. If you buy from someplace like mboemparts or parts.com or your dealer, they will sue your VIN to ensure the correct part.
#4 - I have a backlight fade on the Cluster Information LCD that fades to the top and bottom of the LCD.. can this be fixed with disassembly but without replacement or is the replacement of the LCD the only way to fix the backlight issue? Subsequently where would I purchase this to verify it is the German Made variant?
You will have to remove and lightly disassemble the cluster to replace the LCD screen. This may help:
W203 Cluster Repair
You only need to go through the first 10 photos for screen replacement. I do not believe there is a German supplier for this - they are all from China. usually about $75.
#5 - Headlight adjustment - Previous owner installed some new Xeon headlights and while they work amazingly, they are pointed way to low, is it possible to adjust the lights up? As another related question, is there a good service manual that I can buy that would cover the more intricate service as the manual is severely lacking in some of these simple service
Adjustment is usually done via a hex "bolt" head on the back of the headlights. It should be pretty obvious.
For a service manual, get WIS - it's the official dealer's Workstation Information System. Ebay for about $30.
#6 - What Year wheels/rims are compatible with the 2003 C320? This car has aftermarket (18" Drag Brand) wheels and I am not a big fan and would like some stock wheels. Completely optional and if I have to live with them I will but would prefer the stock (Anyone in the Dallas area want to trade? lol)
Sorry, I'm not a "wheel guy" so I'm not sure what offset you need, but some of the others on the forum are well-versed in all that.