Thanks for all the info. I am in New Zealand for only 10 months -- I usually live in Victoria, Canada and all my tools are at home:( This would be a limiting factor in a complete DIY rebuild.
My plan was to pull the gearbox, clean it up on the outside, open it up and clean up all the sealing surfaces (no gaskets as far as I can tell) and then take the package to the mechanic to do the pressing and setup of shims and whatnot. However, I don't want to wreck any diagnostic info in doing so -- sounds like that will not be a problem.
My mechanic will replace the two sets of syncros and the main bearings. He said that after the bearings are pressed off, they still look and feel good, but he has had rebuilds come back after a short period with failed mains when they reuse them -- figures that the press stresses them in some way.
I have only a basic set of wrenches here, a hammer, few screwdrivers and for some reason the house I rented has an excellent floor jack and, I just found out, a pair of axle stands! I like messing around with my car and I like to be independent: so far I've rebuilt both front driveaxles (including outer CV R&R) and replaced the steering rack. I have no concerns about pulling the gearbox. FWIW I've found the subaru to be a well put together and sensibly engineered car. My vehicles at home are Mercedes and I would NOT say the same about them. But they are interesting to work on!
I'll keep you posted on what happens. Thanks again, and feel free to offer up any other advice that seems to have been missed. Chris