Any particular reason why most people go for 98+ blocks?
 
I've heard that GTX3076's feed a 5SGTE pretty well. Will have to do quite a bit more research into this though.

Does anyone know whether there's a difference between the Gen 3 and Gen 4 cylinder heads?

Yes. Gen4 I presume you mean off a caldina. Doesnt have provision for a dizzy. Has a crank angle sensor and camshaft to suit.

Intake and exhaust ports same as Gen 3.  Castings very similar, appears to be slightly thicker between the water galleries in the head and the combustion chamber which is initially why I bought one as I fitted super large valves and wanted best chances of deshrouding possible without causing weak spots in the casting.  As Just ace mentioned, no dizzy, instead has a machined port for a crank angle sensor which reads off the rear of the intake camshaft.  Can use normal 3SGTE cams however need to modify intake cam to fit the trigger lever. The cam cover is also different.  Not sure if the Gen 3 is similar but the caldina cover has the rear timing 'plate' cast into the cover instead of having a seperate piece of sheet steel.

98 5S block (in US terms) supposedly had more meat/reinforcement around the water pump. Although I've yet to hear of any cracked 5S blocks.

Nice, it's good to hear I can just grab any old 5S.

I was contemporary with the idea of using a Caldina head to gain COP ignition. It's starting to sound like more work than worth.

If your going aftermarket ECU (which you should) adding COP is a walk in the park and doesn't "require" a Caldina head.

True, just buy some toyota echo coils, make a plate the mount them (1 hour) wire the coils in and off you go.
You can wire into the stock cam angle sensor for signal.
Keep your exiting head. The seat of the pants difference is negligible.

in AUS, the best block to get is the 96-98 st204 celica block. has all the reinforcments/redesigns but doesnt have the balance gear holes to block off, has the beefier main caps etc

It does have the beefier mains hey?  I was starting to think all the AUD 5SFE's had the less beefy main caps.  All the US 5S engines seem to have the beefier items.