If you ever plan to tour with anyone else who might be riding a more modern bike - Since you're buying some expensive parts new anyway, my suggestion would be to really consider adding a little more to your budget if you can and get some 9sp shifters, then get the matching derailer and cassette. My riding partner rides 9 & 10sp(two bikes). I made the mistake of going 8sp. Those closer gear steps on his cassettes allow him to ride more efficiently and I end up lagging behind sometimes. I hit a spot where my ideal cadence is between two gears and I have to drop my speed with the gear change. He has that middle gear and slowly rides ahead or has to ride slower for me.

If you only ever plan to tour alone anyway and 8sp will be fine for you, pretty much exactly what bradtx said. Basically anything above Deore level is gonna be just great. I'd actually try to spend more on the chain and cassette than the derailer if it were me. Any decent derailer is gonna shift just fine. I've read that chains and cassettes really do last longer if you get higher quality ones. In my limited experience, it does seem to be somewhat true. My XT chain and cassette on my MTB are lasting quite well. My mid-range SRAM chain on my road bike(which sees less dirt, grime, and torque from steep climbs) seems like it's stretching faster compared to the number of miles on it.