Last year I went on an ebay buying spree of vintage 8 and 16 bit computers. I think generally the cheapest I typically found were for TI-99 4/A’s.

However, I would agree with the other suggestions to buy a Commodore 64, if you are looking for an actual vintage system, that is. This is still such a popular platform, with good parts availability, lots and lots of documentation, active communities, etc. You mentioned BASIC, and while it might not have the very best BASIC, there are lots of books and online documentation and countless thousands of BASIC programs written for this platform. I think overall it is just the most sensible starting point.