The only real diffrence in the GTZ and the ES V6 is the suspension and the braking system. There are also a few other "options" included like the sunroof and the spoiler but that is just the main diffrence.
As far as getting a 98 I am really not sure what you can do with the 7G to mod it. If you can get the 99 and up (8G) then I can tell you what you can do to it to mod it and what will be soon available.
The main diffrence in the I4 and the V6 IMO is that you will get more room to work with in the I4 than you will get in the V6. If you wanted you could fit a twin turbo into there and still have plenty of room to work with it.
Jason