Well, really, like anything, it all comes down to what you want to do.
I've used Amigas for more years than I'd care to admit, now... And, in my opinion, for nostalgia sake, nothing comes even close to using the actual hardware. But, nostalgia, and the occasional educational project, are the only reasons I fire up an Amiga, anymore.
But for running a BBS, UAE should be a great solution. On very low-end PC hardware, you can emulate a BBS faster than most actual Amigas could run it. The CNet I was tinkering around with ran beautifully in a low-priority WinUAE session on an old junkbox (AMD Slot-A 600mhz) even while it was running a bunch of other tasks under Win2000. So for the price of an Amiga ethernet card, you can have a whole UAE system capable of running a BBS.
Plus, you don't want to put classic Amiga hardware through that kind of abuse, anymore. If you want to run a BBS 24x7, I'd highly discourage you from using classic hardware... Especially an A4000. They never were that good at durability and heat management, and over the years they've surely gotten worse. They just can't stand up to that kind of heat and stress so well, anymore.
As for looking for a PPC solution, I don't think any of them will run a currently available BBS.