1) What special tools might I need?
I didn't need any when I did mine a few weeks ago. However you will an assortment of regular mechanics tools beyond just a wrench set. Maybe the hardest thing for me was getting the toothed pulley off the crankshaft - you will probably need some kind of gear puller. I used Beav's method of getting the seals out that requires a pick (search for the text). Other than that I think I just used wrenches, sockets, a large screwdriver (to stick in the flexplate - allows breaking free the bolts on the crank & cam pulleys). A torque wrench is essential to the reinstall - it has to go up over 100ft-lbs for the crank bolt..
2) What else should I replace while I'm doing this (besides the cam seal, timing belt, and water pump)? My car has 135k on it, so what would be good to replace for the near future?
There is a variety of threads on this and opinions vary. you have hit the minimum excepting the crankshaft seal. some also replace oil pump seals, or even the whole oil pump. I think you will need new gaskets on the timing cover itself. This isn't an oil seal, but more in the nature of a dirt/dust seal for the timing case. it will be swelled from the oil leaks.
3) Does anyone remember what the cost of parts may be? Any ideas on the cheapest places to get them (besides subaruparts.com... everyone knows about that one)?
SubaruParts has some of the most competitive prices, but there are others online as well. For the Eastern US there's:
http://www.subaruofgwinnett.com/en_US/