First thing if you are brand name crazy you will spend alot of loot.  Most people who want to add a cheap system are more than happy to go with a small sub and box with an amp.  Lets start with the sub, in this case a mild brand name is a good choice be sure to get the RMS value the sub can handle and then try to purchase an amp with RMS output at the same as the sub or close to it.  To low of amp wattage will cook the sub as well as to high. Ensure the OHMS of the sub are what your amp is looking for as incorrect OHM values will fry the amp, thats a fact!! As for amp let me tell ya Ive had all the names and all of them worked well, be sure the amp you want has a built in x-over so as to only deliver bass tones to your box. I see lots of LEAGACY amps on ebay, good starter choice of amp!! If you want to power only 6x9's no x-over needed.  As for the deck, you can run an amp and sub off of your FACTORY deck provided you shop for an amp with HIGH LEVEL INPUTS(these allow connection of your factory speaker wires directly to the input of the amp) and in turn the amp converts this signal to RCA level internally. I hope this helps a few people out.  There is much more to be said but not enough room here to write it all.  Contact me if you wish further advise via email.  I have over 10 years experience in car audio, and i Consider myself the king of cheap pounding systems!!! PLEASE GIVE YOUR PERSONAL EMAIL ADDRESS IN YOUR QUESTION AS ebay STOPS MY REPLYS TO YOU GUYS SINCE WE ARE NOT REALLY DOING BIZ WITH THEM.  ALSO PLEASE VOTE FOR MY GUIDE IF IT HELPS YOU OUT IN ANY WAY!!