..USDM harness = ~$750+ (best discount I've ever seen, I don't think an employee will get much better than that) and Aristo harnesses are almost given away by the MKIV people who've gone N/A to TT.
There are only a handful (~20) of wires to worry about to integrate into an MKIII. If you want to spend the money right, then buy an Aristo harness (~$20-$100) depending on fool selling it (I got 2 extras for ~$20 each).
Then, send to Dr. Tweak (here on the forums) and you will have a plug-n-play harness for your swap for ~$500..... I can't speak for him, but the reviews I've seen mean that it's guaranteed to work right. I've seen his auctions on Ebay and it may even be cheaper (and he's a stand up guy).
ECUs are fairly interchangeable and a US one will cost you $200+ (Aristo, maybe $100 at most... I got 2 of 'em for about $20 each)
Aristo harness, ECU, and Dr. Tweak to configure it right (for your swap)...easily around $600 tops. Buy a USDM Harness, ECU, MAF, etc...$1000+
Extra ~$400 can go towards single kit, IC, FPR, silicone coolant hoses, motor mounts, etc, etc, etc