Welcome to the boost club. My advice would be wait for the 250 lump. The 225 engine is a complete lottery, unless you know the previous cars history. You could say the same about the 250 engine but unless it was a rep mobile or high mileage, you know the maximum it will be is 3/4 years as oppose to potentially 8/9 years old.
The 250 engine is good for 350bhp standard which would cost you the equivalent of 3k+ to forge in a 225. its not about more power cause you could still run it at 250 bhp standard, its about safety margins. If you did want more power in the future, you know the engine is capable of managing it without up-rating the internals. Its worth remembering that the 250 turbo is better too, and all those who have done the 225 conversion have generally changed for the 250 unit. This would be another £300 on top of what you paid for the 225 engine, and if your lucky, it may come with the 250 engine.
Good 225 engine is a grand, 250 engine is about £1500, and there's one on Ebay now for £1250!!!!!
The gearbox change is personal preference but I'd say wait again for the R26 meg box. The Clio box is great but you need to consider the issues that already exist with it, plus you'll be putting almost double the torque through it. Also unless moneys no option, your talking £1200+ to fit a diff. Andy on here changed from the Clio box to the Megane cause of issues around not being able to up-rate the clutch (?) I think. I'm sure he can cast more light on that. The R26 box comes standard with diff and are around £500, and with the gear link conversion (£75), is pretty short shift anyway. Also the gearing makes it a much more pleasant drive on the motorway....
I can only speak from my own experiences but I'm no mechanic or auto electrician either. I think people forget bolting the engine in is the easy bit, getting it to run is something on another planet. Its this that makes the conversion difficult and been the barrier previously to why these weren't done 8 years ago. Despite the perception, its still not plug n' play.
Cant deny RST are superb and did Jonesy's which was one of the first in the UK. I've used them myself previously and cant fault their work. Paul and Specky are good lads as well. I must say though you should also consider SteC. He's done mine and most on here, and has made this conversion affordable to most through his development work especially around ecu's.