1. You can go for 2 sets of speakers i.e. 1 from each set for one channel. cant say anything abt types of speaker sets u need without knowing where exactly the speakers are placed & the sizes. ex; there are 9x6 full range ones/ones with tweeters, 4' 6' 8' round ones so it totally depends on how you wanna hear things.. just for fun Google harman kardon speaker plans on mercs bimmers etc.. last week i traveled in the back seat of a 7 series for few hours... man its like an orchestra, from the crappy radio to 320kbps mp3 to audio cds everything sounds perfect & acoustic. its a BJ for ur ears
2. consider the Brand, depth of the drive it it is bass oriented. frequency range, max threshold, pmpo n all..
3. One sub is enough unless u gonna hire ur car for outdoor parties. put a 12' sub
4. Most of the setups nowadays outs 2 channels of 50W or more.. so that is more than enough for inside the car & the sub normally comes with an inbult amp around 300W or more.. so that is well enough. dont waste ur money on seperate amps.
Installing a sound system is an art machan.. simply fixing the setup and the speakers will not do any good. u need to place the speakers.. match the frequencies, balance sounds to get the max of it. so get it done from a professional.. otherwise ur car will sound like another 3wheeler.
P.S. There is a JBL Speaker sale at Abans u can get a good pair for less than 10K.. not sure abt the closing date so hurry. ill post the ad here if i can find it somewhere.