pretty much all the brands supported on this forum fit those guidelines. Check out DC audio, Sundown audio, Digital Designs(a little more mainstream than the rest), Incriminator audio, Fi audio, audioque, and Ascendant Audio
i would vote for the cvr's. 2 15's in a trunk would bump much harder as long as you have enough room for a proper box but in a 5 series you should be fine
cops around my city are always on the lookout for anything they can pull you over for. whenever i hear my radar go off i turn my music down until i find the cop. if i ever get pulled over for bumping though ill probably ask to give him a demo
I am a fan of forward facing box sealed off from the trunk but it doesn't really matter either way. for hip hop and dubstep i would say tuning your box around 30-33 would be good. for port area just use between 12-16 sq in of port per cube
when you do the aero ports you need to make sure you need to have 4" between the end of the port and the wall which you wont be able to have with a 25" long port within your dimensions
put your box dimensions into torres calculator then for example enter 10 for height and 3 for width and then enter different lengths until you reach your desired tuning.
the problem with using different size and brand of subs is that youll have cancellation at certain frequencies. have him do what mbdblue said and he'll be much happier in the end
i normally try to keep tuning around 32-35 hz for music but i know some people like higher and some people go lower. if you want to try some different options then use a pvc or aero port and just cut different lengths and see which sounds best to you