If you plan to keep the car for many years and have $10k plus to spend get an RB30 from Rips http://www.ripsltd.com/ for example:
RB30 Stock internal bottom ends, with RIPS modifications, fully refreshed NZD$3,500 rwd or $4,750 4wd including adapter plate kit (suit up to 600hp and 7200 rpm) which is half the price of the rebuilt engine quote you got above
or search the RB30 threads for a reliable aussie builder. The extra torque will make your Stagea pull away hard from idle and depending on what you ask for it will support 300-350awkw easily (or more if you're keen and rich). You will need a decent front mount and turbo etc as well as an e-manage or chipped ecu or better still get a GTR box and a Link computer. Its better to think where you want to go rather than go through various stages incurring more expense at every point (like I did!).
Don't forget to budget for GTT brakes (or better) and some decent suspension. Read the Stagea section and do a search for Duncans RB30 powered Stagea.
If you decide to go for the RB30 PM me and I will give you the full details for my (almost finished) RB30 build.