Im pretty interested too on this topic, because one day i will either go a 3S or SR20 in place of the 18RG..
it has been talked over many times but its not often ppl want to share what needs to be modified in detail because its half the fun of doing the swap: finding out how much of the original engine bits you can use and what is useless..
me and my mate roughly priced it up one day and the SR20t
seemed like an easier bang for buck swap, because of the rwd manifolds & gbox.. but who wants a nissan engine in a toyota if you can have the mightly GT-4 motor in there...
recently been told that a 1JZ would be an easier conversion than a 3S-GTE so that's what it's looking like now