Update:
I found the error. I used the car trunk to put my I items I do not want to carry. After removing them I could buy the car without black screen.
Smitty still speaks sometimes only with me if I save an reload the save game. But I'm mobile, now.
I had another complications:
I also had some things in trunk, so I get them back to inventory. But to have an option to buy the car, I needed to have 10000 (not 5000), although the price is 5000. After choosing buying the car, a still had blackscreen, and 5000 gone.
Finally a managed to get out of it: I saved the game in other slot and exited to windows. After load, I had to buy car once again, but now it was OK (needed only another 5000), and no black screen, car is finally mine
(Win 7 64bit, played GOG version, MM 2.45.4, Mega vanilla weapons, HiRes patch)