Do not flatter yourself: the alleged "relative dilapidation" may well turn into "complete rubbish." Even despite the "thick iron". Restoring the body completely "in the original", having a not too damaged roof and top of the pillars, is a very thankless task.

What are the most important details to pay attention to when buying such an old car?

For everything, absolutely!

If I suddenly decide to sell it,

They buy such a car for themselves not in order to sell it later. Of course, if it is a conscious and hard-won choice...

Approximately how much will a restaurant cost?

In the complete absence of free money, time and divorce from his wife - if any ...

Once, in 1989, a Moskvich-401 had to be restored for a friend, who had arrived back in the capital from a vacation in Alushta, practically "without front spars and struts." He was not a "complete humpback fan", he just needed a utilitarian car for every day, and this Muscovite was once left as a legacy from his grandfather. When we began to disassemble and revise the body, both of us became "quietly ill": there was no point in restoring the cracked parts of the body - they would burst again in the same places or next to the welds. And the reason for this was the recent replacement of the MZMA-402 lower-valve motor with the MZMA-408 upper-valve motor - it is almost twice as powerful. Naturally, the speed has become higher and the dynamic loads have increased, while the roads have remained the same.
An "inadequate decision" was made: we cut off the floor with spars, leaving only the racks and thresholds. We weld a frame from a rectangular box 60x40, which completely repeats the spars and takes on the bulk of the load. As the frame was being made, the question arose: should we slap a checkpoint from the Zhiguli? No sooner said than done: I made him a faceplate at the factory for installing a gearbox on the AZLK-408 engine, since then he already worked "on the coordinate" himself. And the cardan shaft was altered to fit in length and connecting dimensions. Accordingly, the gearbox mounts to the frame were made different than on Moskvich. They assembled a frame with a gearbox and bridges in one garage, rolled it on wheels to another garage, where there was a high-lifted body and connected them together. We achieved the normal closing of the doors, put in new windows, painted the welding spots, put on mastic... Everyone who watched this alteration was satisfied, and especially the owner of the car!
The problems began later, when he wanted to install a Zhiguli engine instead of the M-408 engine, but not some kind, but 2106. No, structurally there was nothing complicated or technically unfeasible. They slightly altered the radiator mount, changed the mount of the motor mounts, adapted the new exhaust system to the body - and the car "flew" again to Alushta. The problems were purely psychological.

Upon my return, I listened to his story.
"Early in the morning, after leaving the Moscow Ring Road, two people on the chisel-09 are catching up with me on the highway, the speed is 110. And through the open window they ask," can you go faster?" Shaking the ashes from the cigarette out the window, "but I can ..." I answer, and go into the gap, slightly pressing the gas pedal. After some short time they catch up, now at a speed of 140, and again ask "can you go even faster ". Again the same answer "but I can", again the ashes of the cigarette in the window and again in the gap. Again they catch up, but now they just wink and go further forward. I add a little more gas, catch up and continue to drive parallel in the right lane - at a speed of 160. I hear the question: "what is your motor from?" didn't see..."
I later asked him if it was not scary to "fly" in such a light car with such a powerful engine? "It's scary," he says, "and where to go? I really wanted to" pull "these dudes ...")))

And that's all, having arrived from vacation, he seriously thought about buying a Zhiguli, as purely utility machine. The humpbacked Muscovite has long been standing in the garage, occasionally starts up and "rolls" around Moscow, and then back to the "stall". It is not forbidden to have two cars...