Personally, (as ive recently done it), if you're not doing it yourself - I would take all the parts in, in a box, and just get the boys to fit them.
Without having my manual close to hand, definately all oils/filters/plugs and everything in the normal list - 'by the book'.
Timing belt definately, (opt) tensioner - if there is nothing wrong with it, you dont need to change it, ($338 dollar saving). The 3 smooth/1 cogged idlers can be sourced off the shelf cheaper than subaru. Worth doing. And the water pump/gasket/thermostat, as Rich said, its optional, but recommended. ($125 odd). There is probably more - ill have a think.
Brent.
** Check page 2 of 02WRX's 'whine' thread in mech section to see why idler pulleys should be done. Proof that they do go, and can be annoying.
**