Everything Cacophony said is true. Prepare for it to take longer and cost more $$$ than you plan initially, guaranteed. I don't know anyone who did a conversion who was an exception to this rule, except maybe one or two highly skilled mechanics.
Also, it's vitally important that you source the engine/front clip through a reputable source, like Possum Bourne, MRT, Perrin, or someone else who is proven trustworthy. A newer STi front clip will cost a mint, but will probably be in best shape, and yield better results. But remember, you're still buying USED parts, and the possibility that an unforseen problem may exist with those parts is both scarey and real.
I have 10k miles on my swap with no probs, although I did throw a CEL at 8900 miles. My front clip had ~50k on it to begin with, too. As far as the CEL is concerned, I have a stand alone engine management (Power FC), so I have no clue what the CEL is about, but everything seems fine, anyway.