I'd put aside $7-8k and I think you will have enough ... assuming you're getting someone else to do the work & wiring.
You'd be surprised at how easy costs can blow out - even a 4AGE upgrade cost me $5k although I'm including extra things such as fitting rear jap spec diff, 1 piece custom tailshaft, exhaust, custom baffle & EFI fuel system upgrade etc etc etc.
The 100kw is a FWD engine ... only the AE86 had a 4AGE in RWD configuration - even the AW11 which was RWD still had its engine mounted traversely.
As for performance, there is a bit of difference in terms of acceleration - Riceboy's car is always quicker to accelerate but I'd put that down to my TVIS system only coming in at 4,000 rpm's or so. From then on, they are about even I think - I've yet to tune my car or get better headers etc.
It will cost you more though to fit the red top as you are converting the 100kw to RWD configuration.