The n900 can play certain 720p videos but not the highest quality ones (note that one cannot know until one tries the film). Needs special libraries installed but that is easy to do.

Portrait support is patchy in the sense that some apps do and some apps don't. A few of the don't'ers are key such as email.

Resistive screen if of the original quality (i.e. not a bad refurbished one and that introduces another issue about 2nd hand n900s) is very good BUT one must be used to them and coming from capacitive might be awkward.

Browser (I don't know the one's you cite): the stock browser is remarkably good for it's era but it is, well slightly venerable. But many swear by it. I use opera and whilst some have frequent crashes, I have very few and like it a lot. There is no flash or html5 support. Well, yes there is a workaround for the first but it sucks IMO. The n900's horsepower just doesn't drive the flash experience well.

My guess advice: not for you unless you are drawn to linux, fiddling with tech and setup and a flexible platform (and a great but dwindling community).