You have to remember these planes are being sold at the end of their service life, so they're worn out. To make one air worthy, you really need to take it completely apart, replace all of the over stressed parts, then put it back together. Of course you need to rebuild it. You also need a maintenance pilot to test it after assembly to ensure everything was put back correctly. Would say a "vintage" plane rebuild would probably cost $10M+, and that is if you can find technicians to do the work as well as the service manuals. Most of the spare parts will be very hard to find, and you probably need your own machine shop to manufacture a lot of the replacements.

Now if you buy something like a F-86 that still has a companies that fly it, you could get off much cheaper if you can contract them to do the work. But it will still be in the millions...

These look like fun, but Bill Gates probably isn't rich enough to keep that entire fleet flying.