1. #1 issue -- dont start that car!
The issue is bad gas -- after 3-5 yrs its mostly water. Most likely the original fill up from 3-5 years ago is till in there, and has been topped up or watered down. Drain the tank, and put in fresh gas and a cap full of Marvel mystery oil and Stabil fuel perservative. It will most likely sit some more during restoration or future years.
That bad gas/rust particles can wreak havoc on the BOSCH mechanical fuel-injection unit...its like a little engine with 6 pistons that is built like a swiss watch. Any water from bad gas will seize it and lead to a costly rebuild.
2. 99.9% of parts are available.. Its a German car, they kept meticulous records and every nut/bolt is numbered.
K&K, Millers, Buds, and the Mercedes Classic Center (Irvine, CA) are some parts sources.
3. 7 Days for most parts. Some like the Top Frame, Grille, Sun Visors, Ash tray are VERY expensive.
4. Chrome....if its missing all chrome trim, the Convertible top frame, etc...it can easily go into multi thousands.. Hard top also get lost/separated from cars, so make sure he has it.
Ps, the plastic heater-levers are very delicate/fraigile and brittle....handle with care...as they break and you have to take apart dash to change.
5. I would flush all fluids, change all plugs, points, filters (incl. fuel/transmsision) and look at brakes, change tires (dry rot). The Calipers are most likely seized (even if it moves) and the booster might have leakes if fluid has not been changed in years and has moisture. Plan on new hoses, belts, and a new battery, battery cut-off swich. The clock drains battery, and its a good fix. Also, get it on a lift, rust can hide everywhere: flloors, trunk, quarter panels....point out to owner extent so both are on same page re: $/time
This car is very sensitive to how its timing is set; I would recommend letting someone with experience handle any adjustments to fuel injection, cold start/hot valves, and associated hardware geometry.
I would also recommend a thorough power washing of outside, underdeath, steam cleaning of engine bay and vacuming/cleaning of inside of car. These cars attract rodents/moisture and musty fumes attract to horse hair seat pads. A good cleaning/shampoo of seats/rugs will add to usability. Further, you can track down fluid leaks/seeps easily after cleaning.
Also, DONT throw anything away from this car....even broken, cracked, rusted, distorted parts can be salvaged and resold for $1000s.
I would get his VIN# and send an email to the Mercedes Classic Center email (below); they will email you a scan of the M-B Data card with all engine, drivetrain, axle, steering #s so you know what you have in original colors, parts #, options.
Classicparts@mbusa.com