Just curious... If i were to buy the parts and fix all of the cosmetic stuff, how much do you guys think this car would be worth?
Well right now W116's have passed their low period, and while the later models are currently the preference as they've aged less, that will shift over time as the car moves more into classic status. Particularly as they become rarer the value will go up. They're five years ahead of the W123's. Right now if you want a super-cheap Mercedes, you end up buying a W126 or W123 or W124.
To get some idea, look at how expensive *any* W108 is. My first Mercedes was a W115, and I didn't really appreciate it much then, but now miss it, and good W115's are expensive and harder to find now. You live in a climate very friendly to the car holding up well. I paid $2000 for my '74 - the paint is shot and there's known rust issues in the sills, but I considered it a great deal - then again the economy is making deals fairly commonplace in recent times. I would imagine than once things pick back up again the values of classic Mercedes are going to start climbing again. From my perspective, now is a good time to buy/keep, not to sell, if you can afford to do so. It will almost certainly be worth more as time goes on.
The 1973 has some particular nice things about it - the bumpers, the higher horsepower, the manual climate controls...and they are still not completely impossible to find in junkyards as the older models have become which can be a cheap source of replacement parts. It'll never have the value of the 6.9 and probably the 300SD will remain above it too, but for those who want a powerful original W116 without the high expense of a 6.9, the '73 450SE is the best choice.
Personally, I'd keep it and work on fixing it up over time, but you can probably tell that from my signature. I would replace the whole door rather than trying to use/repair bondo; that'll be a better investment and won't cost much - properly repainting it is the hardest part if you can't find one in the same color. Color is the only reason I'd never buy yours though! I really don't like blue on cars for some reason. It should be pretty easy to find a same-color replacement door though. Even if it needs a respray the original color will be best as the inside of the door, etc. will all look correct without additional effort.
Replacing the fender is a pain on these models because there is a rubbery underbody coating that basically glues it to the chassis - it is very difficult to remove one without damage. Cutting through the glue with a knife doesn't seem very effective. I've heard that what works is heating the seam up with a blowtorch to melt/loosen the stuff, but haven't yet tried that myself. Don't know how to replace it once you're done, either. I've got extra used trim pieces in good shape if you want to buy some.
If you keep it, be sure to join the W116.org forums as an additional resource and a bunch of us particularly passionate about these models.