You will need... (to make it at all reliable)
Rear thrust block if your's isn'y
updated rods (chinese H beams are fine)
updated pistons
either a 16v head... or a 531 with significant work (Upgraded valves/springs/cams/and porting)
Updated intake, stock 8v and 16v are both highly restrictive
T3/T4 is unlikely to cut it unless its a seriously huge one or has a BB center already
MS or elite 550 are both fine, totally a tuning preference
to get the 300-350hp reliably means either a 16v conversion or a 7500rpm big valve 8v build
Trans wise
CD09 will save you a lot of effort over upgrading the T5, otherwise a TKO500 works pretty well
An AW71L with upgraded clutches etc will hold ~275hp with DD reliability as long as your not spinning the motor past 6200rpm much...
Otherwise... 30-40L with custom bell housing will hold 500hp without issue and give you paddle shift
I think there are now options for aftermarket control of the MB 7 speed auto which does 500+ without blinking
As above... your going to spend 2x or more of the cost of doing a 2GR swap for a worse outcome (laggy at best and mehh on further upgrades)
Did you have a preference on 8v vs 16v?
8V auto/manual or 16v automatic are the easy builds in a 240, 16v manual means upright engine or clutch master in the cabin