If you don't wanna spend big bucks, I suggest you stick to a single woofer, as amplification will be expensive.(I am guessing your car is a hatchback. (swift?)). and one sub will be enough.
If going for a single woofer then the JL Audio 500/1 v2 is a good idea.
JL Audio 500/1v2 - JL Audio 500 Watt Mono Amplifier
for 2 woofers i would take a jl audio 1000/v2
JL Audio 1000/1v2 - JL Audio 1000 Watt Mono Amplifier
Pioneer makes good H.U's which are VFM, but if you are more S.Q oriented get your self an alpine 9887 or even an eclipse cd7200
Eclipse CD7200 mkII - Eclipse In-dash CD receiver which comes with built in BT i think.
Do not forget to get a good amp kit for the amplifers.Get kicker. I have got a kit from u.s and Anwar the installer from Bangalore said, the quality was great.
here are the links. For the mono you will need a 2 channel amp kit
Kicker CK4 2-Channel Amp Kit 05CK4 Car Wire Harness CK 4 gauge gage gouge Audio/Stereo
Also buy a 4 channel amp kit as you are ready to change the sony amp and go for a new 4 channel amp
Kicker CK44 4-Channel Amp Kit 05CK44 Car Wire Harness CK 44 gauge gage gouge Audio/Stereo
If your bro can carry more weight, better to buy some damping also.
Dynamat 10455 - Dynamat Xtreme Bulk Pak 36 Sq Ft
Good luck.
EDIT: I think the amp zak is talking about is the alpine mrd- m605. I have one and its great for the money. Very powerful for a single sub.
Alpine MRD-M605 V12 Digital Mono Amplifier. Best Buy on Sale: Alpine MRDM605 - Compare Prices, customer and user reviews, Features, Accessories, Specifications
I strongly suggest your brother call the concerned online shop, before buying anything from them, as many times the products are not in stock and they do not ship immediately. They also insist on an U.S credit card with both the shipping and billing address as the same.