S4/5 engines are probably the easiest, but Cosmo or S6 13BT's can be made to fit as well, it just means a bit more fab work
Get a S4/5 13BT block, change the front cover to a 12AT one and the sump to a early 13B sump and the rest is fairly straight forward. Use the S4/5 turbo gearbox as well (or a supra box) otherwise you will find that you will be replacing box every week
Custom:
engine mounts+Xmember
g/box mounts+Xmember
new tailshaft will need to be made
diff upgrade
fuel set-up upgrade
and don't forget suspension and brakes :lolcry:
For reliability you are better of starting with factory turbo 13B (i.e. the ones i listed above) as they have a lower compression ratio to suit boosted applications. As well as better oil/coolant lubrication throughtout the block. Many people have successfully turbo NA blocks, but due to the higher compression ratios care need to be taken with tuning and amount of boost used. starting with a turbo block leave far me apportunity to upgrade all "bolt-on accessaries" later done the track when you feel the need for more power.
Ryan