C64s seem pretty cheap on EBay, Atari 800XLs seem a bit more (I don't know you budget save that it's likely "no budget"
).
I wouldn't go with a Atari 400 or a VIC, frankly. The keyboard on the 400 would put me off, and there's simply not enough characters on the VIC 20. The Atari XL is nicer than the 800 simply due to size.
Personally, I would go with an emulator, and work with the discipline of trying to ignore the internet.
The simple truth it you'll likely be using it anyway to look up memory maps and other references, I can see the keyboard being a bit of a pain for a C64 (with all the special characters), but I think you may be rather spoiled by modern hardware and learn just how SLOW these relic machines are. It's all been incrementalism over the years, to where we're pretty jaded these days.
It can be fun to pound on the old machines, but doing anything serious, I dunno. With their crummy keyboards, no rollover, terrible ergonomics, and slow (really slow, did I mention slow?) performance. Anything more than "10 PRINT "Hello world" : GOTO 10", boy that's challenging compared to a real editor with responsive mass storage, multiple windows, cut and paste, etc. etc.
I recall C development on the Atari 800. Whoo boy, that was miserable. In contrast using Action!, which was blazing, and very usable, but partly because it never hit the disk. But those are "fond memories" clouded by time. My assembler assembles my Forth in about 2s. I can't imagine how long that would take on a real machine with real floppies and no MHz.