go the 25, they are awesome, ive had two RHB20 powered cars (and my gf has a stock 32 so that makes 3) and now ive got a 25 in my 32 and i love it!.
You have several options, one is to put ALL the RB20 sensors on the 25 (CAS, knock sensors, ignitor, etc etc) and use the RB20 loom and ECU, remap the ecu and your laughing.
other option is to use the 25 sensors and 25 loom, and still with the RB20 ecu (remapped and tuned to suit of course) with fuel pump, fmic, exhaust you could expect to see 200 + rwkw with a stock turbo on under 14 psi (like 12 psi) the stock ecu wont do this.
as for the box... there is no strengthing the rb20 box "cheap" unless your putting straight cut gears in it.,.. however if you plan on keeping it around 200 rwkw and driving sensibly theres no reason why it shouldnt last. with some drift and track work it will soon die.
Cheers