I was tempted to pickup a sonnet 7200 to start cheap, but sadly they are now insane to pick one up for the limited update they give. Spending north of 1k for a PPC should really be worth it if I'm going to go the Cyberstorm route.
The Cyberstorm/Blizzard PPC cards are pretty slow - WinUAE running OS4.1 on a modern PC is faster - in fact, if you ignore 3D performance, WinUAE even beats a SAM460 on a fast i7.
I've been tinkering with OS4.1 a lot under emulation (testing the latest WinUAE beta) and it's nice - I've been running TVPaint, PPaint, SketchBlock, Cinnamon Writer, Ignition and AudioEvolution and you can do *a lot* with an OS4 system and yes - it's much quicker (on a fast PPC) than say an '060.
What it isn't (so far as hardware's concerned) is good value for money - and the Vampire recommendation is a good one (since OS3.9 is still very capable and supported by Amiga devs).
If you really want to tinker with OS4 (and you've got access to a decent PC) I'd suggest picking up Amiga Forever 2016 and a copy of 4.1 FE for classic as the cheapest way to test the waters.