lots of miss-information here.p.p.s. welcome to the family... quite a step up in the ranks, from a hocus poke-us to an IS... probably one of the best decisions you could have ever made if you plan to mod heavily!
Stock internals are good to 600+hp. Some have been known to push more, but most people modding that heavily opt for upgraded internals, not because stock can't handle it. Some have higher mileage cars and prefer to have the piece of mind and others just have bigger goals.
Stock tranny's depend on which type:
5spd = 350-400 (a few have run higher than this, but only VERY few)
auto = 450+ (they are fairly easily built to withstand 600+)
as far as kits... just read through the links in the thread I posted a link to above.