I am assuming this is going into a celica?
I am also assuming you are going to go with a stand alone?
If you go with a Gen 2 engine, you are going to need a standalone to make honest power over 300whp.
If you go with a Gen3 engine, you could do it on the stock ecu, but you are not going to be able to beat on it like a rally car or track car, or you will surely blow it up because of the "agressive maps" which I will call poor tuning.

JDM Engine/trans/ecu and harness Gen 2 3SGTE will run you about $1200 + shipping at the very least. For low mileage, clean engine. 225hp

JDM Engine/trans/ecu and harness Gen 3 3SGTE will run you about $2200 + shipping at the very least. For low mileage, clean engine.

If you can find these engines cheaper, well good for you, but this is the going price. These prices probably do not include the mounts or needed shift linkage or needed throttle cable. So you will either need to buy these new, find them luckily used, or just put your money into a clip. Good thing about a clip, is you can always sell the shit you don't need. Since you are doing FWD an MR2 long block will be the most cost efective, aside from having to get mounts, axles, shift linkage, and throttle cable. Your other option is to get a celica 3SGTE 4WD clip, sell the transmission, and use your current 5SFE transmission. In my opinion, this is going to be the most cost effective way, but what do I know.

To reach 300hp regardless of a stand alone, you will need some basic mods, intake, upgraded intercooler, exhaust, downpipe, and ebc.

On the Gen 3 you will not have to upgrade anything else but the fuel pump. You will probably even make very close to 300whp on stock ecu and turbo. With a stand alone you could easily acheive 300rwhp with just these mods. The only 'reliability' issue will be the turbo, it will wear fast at 16psi daily beatings. I drive mine at 10psi around town to try and extend the life.

On the Gen 2 you will have to upgrade the entire fuel system, rail, injectors, fpr, pump, lines, etc. And you will have to get a stand alone. You could aslo upgrade to a "ct20" from a gen 3 but even with a stand alone you will not make more then 275whp. So to realistically make 300hp you will need to upgrade to a t3/t4 size of a turbo. The is starting to step away from reliable and towards a money pit.

The head design on each of these motors is like chocolate and shit. That is the main difference in the power levels.

My cost for this would go something like this:

Gen 3 3SGTE 4WD clip $3800 shipped.
Stand alone and tuning $1500.
Exhaust $800
Intake custom fabricated $100
downpipe $300
walbro pump $100
front mount intercooler custom fabricated $800
quality bov $250
ebc $300
stage 2 clutch and lightened flywheel $800

All these parts new except the engine of course.

sell awd trans and other misc crap from clip -$1800
sell 5sfe -$250

I mean you can get some of these parts used, and use a isuzu intercooler and yada yada yada but this is how I would do it, if I ever where to do this.