Not sure why you think top mount is better for your application but I'd suggest looking at FMIC as it is a nice simple system. Also I'm pretty sure that RC or ST205 WTA doesn't support 550hp.
I'm going to be straight up honest. You want a beast yet you start with a freaking heavy car with a very small aftermarket parts bin. You chose the wrong platform IMO. A WRX or an EVO would have been a better place to start. Even Supra's have bigger aftermarket parts bins than us! Don't forget, its a 20 year old car with an engine that was limited to a couple of platforms.
You want to hit 500 - 600hp, and that is a lofty goal. So I hope you know that it isn't cheap to do that. Be prepared to pay $$ to get that power, just like with any other car.
Engine mounts: Only need front and rear changed over. Can add dampers if required: http://www.speed-source.net/
Fuel Rail: Wolfkatz or http://www.speed-source.net/
Radiator: ASI 56mm - ebay. May need low profile fans like Spal to increase difference between fan rear and turbo
Cam Gears: HKS, C-One, Generic. I have C-One cos the HKS look ghey haha
Cams: Go 264s or 272s. I'd suggest 264s as they have better drivability, HKS or Kelford. Read this thread and look at cj motorsports test results
TuningBelt: HKS or Gates
ECU: Take your pick - I always suggest Link as they are local to me. Could go Motec if you are ultra serious but overall whatever suits your flavour and has the functions/datalogging you need.
Turbo: Take your pick. It better be big. Nothing else is a bolt on in the Celica engine bay and will require an adaptor at least or go with a new ex manifold. Many people using custom ex mani's to suit the turbo they want to mate with their engine.
That will start you off. Until you blow the engine. Then you will want to look at st205/gen3 or st215/gen4 engines. Then you will also throw into the mix a hybrid engine 5sfe using 3sgte head commonly referred to as 5sgte.
Read here, read the UK forums, read the MR2 boards.
Either commit or try a diff platform but I reckon that 500 - 600hp with limit budget will just cause heartbreak. The celica's a whale and most owners have a love affair with them (like myself) so be prepared for a not so easy modification path.