My two cents here. An 8sp setup can be run with a 7 or 6 sp chain. I'm not sure if this can be done if you go with 9sp. All this at the expense of having a short gap in your ratios.
Paul thumbies are pricey, but don't expect to get NOS XT or in good condition (which I consider essential of an expedition) thumb shifters for a lot less. I know because I went that route and ended with Paul's, with the added bonus of having the possibility of using downtube shifters instead of barends. I'm not sure if it's easy to get SunTour 8sp shifters out there or even make them work with a Shimano derailleur in SIS mode.