I would add, "There is no such thing as a cheap Mercedes".
Along with Brad's comments, you should always buy at the top end of your budget. It's far, far cheaper to buy the best possible example of a car you're looking for than to buy a "project" car, or one that requires a lot of work, especially if you expect to drive it regularly.
And I'm speaking from over 30 years of experience in this area, specifically with Mercedes. I learned my lesson early in life, thank goodness, and have applied that knowledge ever since with great satisfaction.
Don't fret about parts. I have a 1965 220SEb, and the parts that are still available from Mercedes is nothing short of amazing. You'll find much the same with the W108 chassis as well.
Never, ever let someone tell you a part isn't available unless you've checked with the Classic Center parts search. That's the end of the road - if they don't have it, you're not going to get one (for OE parts, that is.) Much of the maintenance items like soft parts and shocks, for example, are all over the secondary market. Stick with the OEM manufacturers like Mann, Hengst and Bilstein and you'll be fine.
And also check prices on parts from Mercedes - first! I say that because there must be a significant number of idiots or poorly uninformed people out there paying stupid money for used or new parts that can be had right from your dealer for less. For example - the tristar on the trunk lid of a W108/109/110/111/112 chassis car is under $50 from the dealer, NEW. Go take a look on places like eBay where sellers are putting used and new ones up for sale for twice that. Whether or not they're selling them, I don't know. I'm just incredulous that there is even a market for them since the dealer is the best place to get one....