The first thing to realise is that as you wind power up, you will lose reliability and lifespan, especially on an old engine. Too many people start turning the boost up, bolting on goodies and then get upset and scrap the car when it goes pop. Not wishing to put you off, but just go into it with your eyes open - I've seen too many people spend loads of money and then get disapointment.
Don't skimp on supporting mods, especially cooling.
A FMIC is one of the most important on a 185 - you already have that
The 185 suffers badly from overheated oil, partly because the filter is very close to the downpipe. There is a kit available to relocate the filter and add an oil cooler at the front. This is very worthwhile, especially if used on track or prolonged high power use.
The standard radiator isn't too bad but if you find overheating problems have a look at a larger aftermarket rad (beware rads for st205 are taller & won't fit without cutsing & welding). Also worth going over to a presurised header tank system rather than the stock expansion tank system.
For people running above standard power with reduced safety margins, I tend to reccomend a wideband AFR gauge. One of the biggest failings of the standard setup IMO is that there is no fuel pressure or temperature sensing. If the pump starts getting weak, or a bad batch of fuel clogs the filter, or even if the fuel gets hot on a sunny day the ECU will carry on regardless and let the engine run lean with resulting piston failure. By keeping an eye on AFR while on boost you can make sure it stays safe. (make sure it's a wideband sensor, not a 'weak/rich' meter which attaches to the standard narrowband. The full kits are normally about £150-£200)
Fortunately for us, the GT4 is fairly unique in having transmission built like a brick outhouse. No real work to be done unless running very high power. May be worthwhile fitting a larger cooler if it is used on track or prolonged high power / high speed.
The standard clutch should be ok if in good condition, but won't cope well with hard launches. For normal road use, the standard clutch will outlast anything, but for competiotion / hard use look for alternatives that suit the usage.
As far as getting 300BHP:
Stock fuel pump on a 185, especially an old one, will need to be uprated. Walbro 255 is a common swap but there is supposedly a better one which I can't remember. Have a look in the build thread by animal66 as he used it.
Standard 185 injectors are 440cc and you'd be right on the limits, better to go for the 540cc from an ST205. They are a straght swap from what I can gather. This would, of course, require an ecu change.
ECU wise, there has been a lot of discussion on here lately about the benefits of a stock ECU and the decision to go away from stock shouldn't be taken lightly as you're walking away from all the safeties and months of mapping done by Toyota. The decision is easier on a 185 as the AFM setup isn't great and at 20 odd years old tends not to be very accurate. For the horsepower you're planning, it would make sense to go for a standalone MAP sensor based ECU and ditch the AFM, making a staight intake for the turbo easy. On the other hand, if you're lucky enough to have one that doesn't run massively rich it may be better to drop your goals slightly and retain the ECU, especially considering the turbo options.
Turbo choice is a bit of a minefield. IMO a standard ST205 turbo is a good choice for road use, but will be on it's limits at 300BHP. There are a load of hybrids available but most are wasted unless you have a forged engine and run 1.5 bar boost. Some are also prone to turbo surge. You will generally also find that you will lose bags of bottom end torque and responsiveness.
Many on here run the turbo technics hybrid with good results, but that is the older single wastegate version that doesn't suffer boost creep. I believe it's now discontinued. I know others who have had issues with reliability and poor customer service.
I'm running the Fensport stage 2 CT20 (equivalent stage 3 CT26) Gives good top end and very responsive provided RPM stays above 5000. Driving up hills in 5th isn't going to happen unless you're doing over 100MPH.
Also worth looking at Garrett. A lot more money as they don't just bolt straight on but there are reasonable kits available from TB developments with all the adaptors you need. I saw andycaca's 165 produce nearly 350BHP at 1 bar boost. I think it was a gt3071.
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