wud have been easier if u cud answer the other two questions
u want woofer?
u dont have any issues with installing an amp?
well heres a possible cheapest decent combination
run stock head unit, ( alpine? )
add alpine type S components in the front,
any cheaper alpine in the rear,
add a 300+ rms watts, ( around 800-1000watt peak in local terms), any brand wud do.
another option incase u need woofer
stock HU
alpine type S components in front
alpine type S woofer
stock rear speakers
run front alpine and woofer with 1000-1200 PEAK watt, ( 500rms ) amplifer.
run rear speakers thru HU
if all u need is SPL and no SQ, go for pioneer stuff.