Basic answers:
Cargo space is great, as already pointed out, and yes, the seats fold flat;
if you want AWD and auto, then an RS-Four is the go;
October 1998 onwards was a series 2 C34, so they all have RB25DET Neo engines (same as R34) - 206kW @ 6400rpm, 333Nm @ 3200rpm;
AFAIK, there's no standard factory cruise control, so aftermarket would be the best bet;
mechanical problems should be fairly rare, but just make sure it's had (and always will have) decent fluids, regular servicing, and make sure the timing belt has been changed;
the car has two front airbags as standard, and I don't think I've seen any C34's with side airbags;
ABS is standard, but traction control isn't (unless that was exclusive to s2's) - it's AWD anyway, so you wouldn't really need it;
insurance is something only you can find out about - everyone is different in this regard, so it's best to make your own enquiries and see what you can get;
oh, and HICAS was only fitted to factory manual Stageas, none of the autos had it.
Have a look at the Buyer's Guide I wrote up ages ago: http://www.skylinesaustralia.com/forums/topic/92065-buyers-guide-the-nissan-c34-stagea/
Good luck with the search - you'll definitely love being a Stagea owner!