If it can run Bad Company 2, it can
probably run New Vegas, though I don't know the specific requirements for either.
You can buy it off of a disc or Steam, it doesn't matter - the
only difference is that with a disc, you don't need to wait for Steam to download the base game and can instead go straight to DLing patches, while if you buy it off of Steam you 'just' need to DL the several-gigabyte base game. There's literally no other difference - even the disc version runs off of Steam.
Downloading mods is easy enough, installing them is trivial, but making them all work right together if you get a bunch of them can be a bit more involved.