Depends. If you are coming from a Unix/Linux world,
*or* started out with
the GWBASIC crowd, a Tandy (CoCo|Model3/4|etc.) might be easier to get
accustomed to as there will be less of a learning curve with the Basic (or
in the *nix case, OS-9 on a CoCo). Parts are still cheap, plentiful, and
some are even swappable with the IBM scene; not to mention utilities to
write to cross-platform floppies & whatnot; which is a bit tougher in the
GCR world.
However, I agree that if it is your first foray into the world of classics,
a small cheap 8-bitter is definitely the way to go.
If you're interested in going that route, the 2 machines I'd recomend
would be the CoCo series for much the reason you gave (OS-9 is an
interesting operating system) and the BBC micor (excellent BASIC and
assembler built-in, lots of standard interfaces built-in, plenty of
software and docs on the web).
-tony