if you are looking for a reasonably cheap way out, i would suggest spending all the money you intend on spending (for now anyway ) on the subs and sub amp. to get a good sound w/o going overboard i would suggest runnning two 4ohm tens and a two channel amp (preferably one that can handle being bridged down to a two ohm load) to get complete use out of your subs. not wanting anything to crazy your head unit and factory speaks should provide decent sound for now.
now as far as the brands, you should really go visit a few stereo shops and listen to what they have so you can factor in what sound you are trying to achieve w/the budget you have set to stay around. make sure you tell whoever showing the equip. what type of music you usually listen to, this aides in picking out a speaker box design/size as well. a few brands of subs that i have experience w/and have had good luck w/are XTANT, Concept, Image Dynamics, Digital Designs, and Kicker. sorry for ramblin on and on. hopefully this helps w/o confusing you, if you have any questions let me know.