it sounds like you know exactly what you're doing as it is :thumbup:
I wouldn't worry too much about valve clearance, etc, but I would quite definately change the belt simply due to its age and the fact that it's rubber. but in the end, the engine is non-interference, so a snapped t-belt won't kill you- it'll just inconveniently strand you.
one problem that a lot of us have though, is taking the differential plug off of our transmissions. if you can't get it off, don't worry about it, and do a couple of drain and fills, since the v6 trannys are actually one connected unit (unlike the 4banger tarnnys with actual separate units).
and relocating the knock sensor is also a great idea- toytota designed the engine overly safe when they first came out with the 1mz. changing the bypass coolant hose under the intake manifold is also a good idea.
as for belts for a better price- try delco belts. oem quality at a much cheaper price (generally speaking)
let us know how it goes :thumbup: