The JDM 4th gen Supras are inferior to the US models. The RZ-S was the top of the line sports version, with an updated TEMS- like adjustable suspension but it had smaller brakes, slightly shorter gearing in the 6-spd (via a 3.23:1 rear diff instead of our 3.13:1) smaller injectors (440cc high impedance instead of 550cc low impedance), inferior ceramic turbine turbos, lower stock power output, less agressive cams, etc.
The only thing nice about the JDM 4th gen Supras started in around 99 with the tiptronic style autos, factory navigation, etc, along with the fact that the vast majority of them are hardtops, which is desireable to me personally from a performance standpoint. Here in the states MK4's got a cosmetic upgrade with a different front bumper, side markers, headlights, and taillights in 97-98, with a more attractive dash and steering wheel on '98's as well. For modding, the best years are 93-95 to avoid the OBDII ECU, in US models anyway.
The VVT-i 2JZ-GTE's in the later years have been proven time and again to be inferior to the previous non-VVTi for modding, as there are no aftermarket cams and unlikely to be because of the VVTi's dependance on the stock camshaft profiles in its stock tune. It also went from a coil-on-plug system, found in the first gen and US delievered 2JZ-GTEs to a wastespark system like that on the 7M-GTE. also, the more advanced ECU its much more difficult to fool/tune for larger turbos, injectors, etc.
As far as importing it goes, because Supras got here is meaningless to the DOT as right-hand-drive cars no longer qualify as "substantially similar" to a US model unless the manufacturer would be willing to send a letter to the NHTSA and DOT assuring them that it would meet all crash standards. i.e. not going to happen.
Do some reading on what's required:
EPA :
http://www.epa.gov/otaq/imports/quikover.htm
DOT :
http://www.nhtsa.dot.gov/cars/rules/import/
US customs :
http://www.customs.ustreas.gov/xp/cgov/import/
So I'd just start saving your pennies and get a US delievered MK4. TT autos can be had starting around $18,000+ and TT 6-spds around $20,000+ Naturally, I think you'd be better off budgeting another $5k or so more than those prices to find a decent one.
Good luck.