why WRX seats? would almost be easier to lurk forums and find some cheap fixed back seats, should be able to pick some up for $100-$200 each and then just mod ke rails to suit.
change your struts to ae86 ones, and buy coilover kit from anyone really (ajps, otomoto etc etc), xt130 lca's or sigma lca's. lock spacer?
hilux diff's are getting up there with being really heavy, and you're not going to be pushing out overly huge power so welding the stock diff should be fine, if you start breaking those opt for a T-series, (from t18, can't recall exact model corona or ra60 celica.) T series also gives you LSD option.