In all honesty, a PPC card is just an expensive toy. Not that the hardware is a toy - they're very good, well-made boards - just that the amount of really good software available to use them probably barely reaches double figures. If you have money to burn then get one, other wise stick with the 060 - in normal daily usage an A1200/060 will feel a lot faster than an A1200 040/PPC.
I've owned several PPC cards and only used them for playing games like quake and shogo (which I could have ran at 10x the speed on the PC sitting 3 feet away), PPC picture datatypes, and emulators - considering the cost of the boards it isn't worth it.
A decent amount of RAM for an A1200 would be 64MB - enough to comfortably run OS3.9 and several applications. If you're going to be working with art packages and lots of high resolution images then you could get more - 72pin simms can be picked up for a few cents these days on eBay. The maximum you can put into an A1200 is 256MB.
I don't think there's anywhere cheaper than eBay to get an A1200. There's Amibench, there's a 'for sale' section here and at amigaworld.net, and there are various online resellers, but eBay is the cheapest.
Nor can I think of a cheaper way to get a pre-modded tower case. You could buy a cheap PC case amd mod it yourself, but you'd have to be pretty handy with a Dremel and it would take a lot of time and work, and still wouldn't look as good as the Elbox one.