@RDS: I looked at a bunch of options before deciding on the V6 conversion.
4AGZE - About the same cost of the 1MZ for less HP and less torque.
4AGTE - Laggy, more engine bay heat, needs good intercooling. Too much hassle
3SGTE - Too heavy, too laggy in stock form, fairly expensive. I also don't like the idea of turbo AW11's, something about a glowing red object generating crazy heat in the engine bay puts me off.
SR16VE - Too much custom work, transaxles, shafts etc etc. Bit of a pipe dream idea.
The 1MZ was a good option for me, it is only marginally heavier then a 4AGZE, provides plenty of torque and hp with brilliant reliability.
@XMR20: It's tight, but it's not as bad as you think. Due to the shape of the V6, there is enough room to work above it and below it, very little amount of stuff has to be accessed right against the firewalls. The only issue I found so far was the oxygen sensor on the standard manifolds won't fit in the engine bay due to position, this isn't a big issue as I'll be making my own manifolds and the oxygen sensors will be tucked in towards the block.
@Vinny: I'm not sure at the moment, once I have fabricated and installed the front motor and gearbox mounts, I'll work on the torque mounts. I have some ideas for semi solid mounts that should only cost me about $15-20 per insert, but it's all to be tested yet. Will burn the bridge when I get to it.