£100 aint gonna get you much in the way of an amp. But for a little more money, you'll get something good! I spent £180ish on an Alpine (many others available at that price point too) - If your upto the install yourself, there are a number of good online ICE dealers (do a search on the forum they've been posted recently). If not, expect to get stiffed for about 2 hours labour on the fitting of the head unit and the amp (£50ish).

Put your name down on the tuner list ASAP too!

Enjoy your new toy when you get it installed, I know I still am!