Sounds like a great deal to me....$350 for the gun and can, plus $200 for the transfer (or is it less since it's not new?). I think new, around here, the P22 goes for just under $300, and the can is ~$250+tax stamp.
Some P22's have issues with the safety walking on during firing. This is an easy fix. If yours has this problem, let us know and I"m sure we can help you out. Some folks also complain that the P22 has a weak extraction. I suggest going to www.rimfirecentral.com and checking out the Walther sub-forum. Read the P22 bible that is posted there.
I've heard a TAC65 with a P22 as a host, and it's very quiet. I think you'll be satisfied with your purchase. I'm in the market for some extra magazines too. You can purchase them online directly from Walther. I imagine if you shop around, you can find them cheaper elsewhere though. If you find a good source, let me know.
ETA: I would make sure you get the key for the ILS, and make sure you can unlock the gun. Then just put the key someplace where you won't lose it, and never use it again. There have been issues with folks locking their gun with the ILS, and having great difficulty in getting it unlocked.