IS200 IS300
Price 20230 26700
Weight 1380 1500
Power 155 215
Price difference is 6470, this will get you:
Super Drager 355
Super Charger 3900
UniChip ECU 550
This should give you about 220 - 230 HP and leave 1665, you could spend this on more engine work (manifolds, boost upgrade, induction kit etc) or work on the suspension:
TTE Coilover kit 795
CUSCO Front and Rear Strut 200
TTE roll bars 325
Still leaves 345, get an induction kit made (say 320) and put the remaining 25 for your TeamLexus GoldMemebership :D
Overall you will have a 230 - 240HP IS200 that weights 100kg less than the IS300 but cost the same.
If you intend to go very high horsepower the IS300 is more suitable than the IS200 since the engine has already been widely tuned.