Why not use the 5M-GE, 6M or 7M-GE ? I would have thought they would be the simplest, cheapest conversions, and a substantial upgrade from the 5M-E ? I don't want to do a 1G-GZE/GTE as it seems to me to be the most messing about of them all, as well as NA engines often tend not to have had such a hard life.
Possibly a 1/2JZ-GE could be considered, but they're a bit pricey.