Only some questions of myselves:
What is the point of using a PowerVR 1 ? It is oooold. Why not use a current 3D Chip ? Well, anyways, if it would be a recent chip based on the PowerVR that would of course be preferred.
Did you check if Hardware documentation for the PowerVR is available anywhere ? If not I'd go for a different chipset (Radeon comes to my mind, as
to whatever you want to go - OS 4 or MOS - both
are supporting Radeon). At least for 3D new Drivers
would need to be developed... not sure if SNAP supports the later versions of PowerVR (and I think the first one is pointless) or not, did you check this ?
You should better check this before limiting yourselves to a specific 3D Hardware.
I do not think using A4000T Tower-case is a good
idea. There are tower-systems (ATX-based) which
are much more professional (as how easy it is
to exchange hardware). A special-boing-ball-wielding
tower

would of course be an idea... but I am sure you could work something out with a tower-case-maker...
BTW: Basically all 3D Chips are "Custom Chips"

If you look at how to access them from the "low level" - without standard drivers - there is nothing "standard" in them. Custom Chips means just "Chip produced for a specific purpose", there is absolutely no "Magic" in it.
I think Zorro should be scrapped. Even Dave Haynie
told this some while ago...
Steffen