HI, If your after just a little bass then a 10" sealed enclosured sub would be fine.
But what i noticed was in an AE86 a 10" sub really isnt enough bass because of the large volume loss throughout the cabin.
I wouldn't reckon using 6x9's if you are using a sub/subs because they distort the low frequencies produced by the sub.
My opinion would be to use 4" splits in the factory location and mount the tweeter abit wider and higher to lift the sound stage, and get a 12" subwoofer in a sealed enclosure and tune it to play vocals, if there's not enough trebel then add a pair of 6" coaxial dual cones to the rear and run this all of a 4 channel amp.
A sound system isnt all that heavy, im running 2x 12" MTX subs and speakers front and rear, and in total it ways 20kg, so not too bad i guess, just keep wiring clean and remove excess to shed weight if need be.
As for the Bostons , i reckon there great, i would also reccomend Pioneer as a bang for your buck components.
Hope i helped ya. Cheers Tai