Swift Need ICE Upgrade – Help Needed

Hello ICE gurus,
The Present ICE set is only the HU – Pioneer DEH 5950 and the frond coaxial Speakers. I would I like change that coaxial speakers to back door and add a components to the frond and add a subwoofer to the rear parcel tray 10” or 8” (Since I don’t want to compromise on the boot space). Now my questions are
1. What would be the ideal size of the subwoofer which fits on the rear parcel tray? ( I would like to add a 10”)
2. What kind of amp should I go for 2 channel or 4 channel ?
I would like to know about the Bull Audio speaker availability in Bangalore. My budget would be around 12-13K. Kindly suggest me the right combination including the brand and the pricing.