4.5 is the One to Own
I agree that these are great cars. Truly an engineering marvel for the time. And fortunately they are an affordable classic.
Your biggest enemy will be rust, both in body panels and the frame. If the car has been repainted I would make certain you investigate all the usual spots very carefully. Repairs can be costly, and clean body parts are hard to come by. I know, I learned the hard way.
The 4.5 engines are unbelievable. However there are some things that are the "nature of the beast".
Expect to get 10-12 mpg.
The fuel delivery/injection system only operates at about 1/2 the pressure of newer cars. With such a thirsty engine, 8 injectors, and an unprotected fuel ring exposed to the engine heat maintaining the fuel delivery system is key to performance. This may be even more critical if you live in AZ or some other hot climate.
I took my 72 280SE 4.5 with 215k to a MB "mechanic". It smoked a little, but ran great. He pulled the plugs and told me I needed a valve job and full top end replacement. Instead I replaced fuel lines from back to front, filter, injector seals, 2 new injectors, and had the tank purged. New set of plugs and $300 it was a completely different car - true Deutscher Muscle!
Unfortunately I did not look closely enough at the body, and after falling in love with my classic, I had to let her go because the body repairs were astronomical. There are many good ones around with little or no rust and I am now looking for a replacement.
Viel Glueck! Michael