that may very well be, but G5's are still not cheap and considering how much more powerful even the first-gen 32bit Intels are (unless you plan on getting a last-generation G5, which are even more expensive), a used Intel Mac is just a more practical purchase at this point.

Now, if Classic support is a must and you don't mind being stuck with Tiger (which is a perfectly fine OS in its own right), then a G5 is obviously your best option.

But realistically what do you just have to run under classic that you cannot find a free or open source alternative which is OS X compatible for? The Apple software market is flooded with thousands of excellent free or shareware utilities which in many cases are better for some things than their counterparts from corporations (insert Linux fans).