Hook up your sub by connecting the "Sub Out" on your receiver to the "low level input" "in" on the sub (check the manual to see if one is preferred for monaural input).
You didn't put a hyperlink to the EMP5.1 system, but I'll assume it's "bookshelf" or larger (not some micro-speaker system that comes with a separate woofer), and say "make sure your receiver has an amp a bit more powerful than your speakers. Consider especially the listening volume relative to room size relative to speaker size (in other words: if you are about to play really small speakers really loud in a large room stop here and reconsider).
After that (enough wattage), you buy a receiver for features. Onkyo makes a fine unit, as do Denon, Marantz, Yamaha, Pioneer, etc. If it has the features you want (enough power, enough inputs/outputs) then you are good.
50' is probably not enough speaker cable for HT, and 12 AWG is *huge* for the distances you are likely to be running. Try a smaller (bigger number) gauge like (see
http://www.roger-russell.com/wire/wire.htm#wiretable) and figure out what footage you'll actually need (if you put one surround speaker in each back corner, and a receiver in the middle of the front wall, and your room is 13' long and 10' wide, and your receiver and speaker are both 3' from the floor, and you will run the wire on the floor, you will need (at least) 3+3+5+13 = 24 * 2 = 48' of speaker wire just for the two surrounds.
