hm i just skim read a lot of this, but it is a little confusing, so il say what i know from the top of my head..
There's the 1st Generation JZS147 Lexus GS300. This car was sold in America as a non-turbo 2JZ-GE. It was sold in its home land of Japan as the Toyota Aristo and i believe came as both turbo and non turbo 2JZ-GTE and 2JZ-GE. i think they were made from about 92ish - 97ish? These are available as imports from Japan, hence any coming in from Japan are Toyota Aristos though some have had different badges put on them. Most that are imported ae 2JZ-GTE because people like the turbo engine.
Then there's the JZS160/161 model Lexus GS300, the 2nd Generation. This was sold in America and in Australia as a Lexus GS300, and in Japan as a Toyota Aristo. Again, in the overseas markets this came with a non turbo 2JZ-GE engine but with VVT-i. In japan it came as turbo and non turbo versions, though i believe they were all Auto from factory. These models of Aristo cannot be imported into Australia since they were sold here domestically, hence you need to do an engine conversion to get a 2JZ-GTE version. they were sold from 98-00, then a face-lifted model with updated taillights and grille came out and was produced from 01-04
You can do engine swaps by using a 2JZ-GTE VVT-i front cut from a Japanese Aristo and putting it into the GS300 but some dash lights etc come up unless you go the whole hog with wiring harnesses and so on.
An NA-T conversion is done alot in america, both on Aristos and on things like supras. Look on clubna-t.com for some info. Doing an NA-T conversion is a good way to get about 300rwhp from the car with the turbo, injectors, ecu, exhaust, and front mount. Any mroe than this required serious engine work, and it would be better to start with a 2JZ-GTE engine for building, at least the head since it is better.
Both of these turboing options are pretty expensive though, like close to 10K.