The least expensive way to try things out is, of course, via emulation, and there's very active work and archiving being done for early 8-bit microcomputers such as Apple II, Commodore 64, TRS-80 and Atari machines. Apple emulators, such as Virtual ][ (http://virtualii.com) and AppleWin (https://github.com/AppleWin/AppleWin) at least, let you emulate not only the Apple II but CP/M as well. There's old hardware out there as well, with all the problems associated with it.
For other old systems, such as the PDPs, Vaxen and others, there's simh (http://simh.trailing-edge.com/ and https://github.com/simh/simh).