It all depends on how deep your pockets are and how much hair you wish to be pulling out...
Stock 1mz will be around 200hp and a fairly pain-free conversion. It'll be quick to do and plenty quick enough to drive.
1mz SC will sting the pocket. The bolt-on SC's from TRD are becoming like hen's teeth to find as they all seem to find their way into enthusiasts hands - then command £1500 at a minimum.
So you could do a Rotrex SC - that's around £1800 to start with - plus fitting piping and cooling of some sort. Plus you'll want Turbo running gear.
Or Turbo 1mz - that again for a low blow, well cooled could see ~300.. Though if going turbo I would suggest a 3vz as they're stronger. There's one in Germany on a stock engine with fibre gaskets pushing out 390whp. (Decent Turbo, proper fuelling and excellent cooling - stand alone ECU).. But you're talking £10k easily.
2GR ...
As the voice of experience.. This is not a cheap or easy power route.
Like you say - engines on ebay USA - $2000. Then it'll be around £1250-1500 to get it to the UK (including import duties)... Not forgetting you'll need an assurance it's a good runner and they'll supply all parts you need.
I can fabricate the engine mounts for the drivers side for around £120.
You'll need manifolds (I have a UK solution for £600 which I am waiting to see the results of or Gouky's for around £800 once imported.
Then a custom y-pipe £3-400
Then custom exhaust £3-400 at least
Installation... If you can do the mechanicals yourself I think you'll need someone to wire it up - 2 days labour I reckon.
I'm assuming you're N/A at the moment..? So you'll need a turbo gearbox (with LSD), driveshafts, hubs etc - £1000
Factor in another £500 for odds and ends (oils, coolants etc).
It soon adds up.... To hair loss I assure you!
Honestly ... what do you want...?
Do you want a little more grunt than N/A but torque from low down?
- Buy a for conversion 1mz as Paul suggested
Have you had turbo cars previously and love the surge of power?
- Buy an MR2 turbo or convert yours to turbo (buy a whole car - cheaper)
Do you want something unique with a cracking sound to it and plenty of poke?
- Do a 1mz SC conversion
Do you want that slab of power from standstill & cannot stand turbo lag?
- It'll be the 2GR conversion for you then sir!
It's definitely a 'horses for courses' or 'strokes for folks' scenario. Plus not just a case of wanting more power. We all (well most) want more power but how it is delivered is more important.