You could work around the issue of savegame backwards compatibility by providing the alpha/beta versions as standalone zip-files, similar to games like Rimworld, so you can keep them in separate folders. Also in already released games like Europa Universalis and Crusader Kings, backwards compatibility isn't always guaranteed between major version changes, but they do provide you with the option to run old versions via the Steam beta tab. I think it just becomes an issue if you forcefully override past files/savegames without giving the user an option to work around it. :)

I think there was at least 2 point seatbelts at the time. Those things existed for a long time.
Wikipedia says:
http://en.wikipedia.org/wiki/Seat_belt#History
[...]while Swedish Saab first introduced seat belts as standard in 1958.[5] After the Saab GT 750 was introduced at the New York Motor Show in 1958 with safety belts fitted as standard, the practice became commonplace.

The car appears to be a Toyota Corolla Sprinter E10, so I assume it has one. :) But I could be wrong, not 100% sure.

I would like to suggest a few features that might be enhancing the game.

First and foremost, a seatbelt feature would be a nice addition. :) Seems like a good thing to have in a game with perma death.

Second would also be regarding said perma death. Have you considered injuries? For example driving recklessly into an accident could injure the driver, especially if not wearing the seatbelt. Then he would need to try to get help, while bleeding all over his nice summer car. :( Example injuries could be for arms, legs or head, reducing the ability to control steering, shifting, acceleration or braking as well as vision (blurry, passing out).

Another suggestion would be a start with a somewhat functioning car that can be tuned or has some issues that need to be buffed out, for lazy people like me that are scared of the idea of spending a long time without any functioning vehicle. I was wondering how you'd go around living without a working car, when you'd have to go out and get parts to build it and also go out to shop for groceries. That start could of course mean less starting cash.

That seems like a very strange pricing policy, but I guess that is okay for finding out what people are willing to pay for it. :)

Hope it comes rather soon and doesn't fall into the "endless polish before showing it to the public" trap. But of course it is understandable to make it really nice before showing it to others and charging money for it.