I can't help with everything but here goes:
As far as processor and memory goes there are two* possible upgrade paths: You could get a 68060 accelerator for more speed or a PPC/68K card for even more speed (but less apps use it).
Both are expensive and can be hard to find.
*) not counting vaporware.
I'm not fully familiar with the Z4 board, but unless I'm mistaken it's a proprietary expansion standard so I'd be wary of using those. PCI slots are probably a better bet. You could also add USB devices via a Subway (IIRC) or similar device.
However: I'm pretty sure* you cannot use an AGP or PCI/Express card in a Classic Amiga at this time. You might be able to find a PCI card, but they do not have comparable performance to the AGP/PCI Express ones.
*) 95%
Hope this helps a bit!