you wont get a decent alarm with an immobiliser fitted for that price. realistically youre looking at least double that for a basic CAT1 alarm fitted by a professional. cheaper if you can get a friend or you have the knowhow to do it yourself.
the best way to protect a car like a 106 from thieving gits is invest in a steering wheel lock such as the disklok.
they can be had for £50 i think and is the best deterrent money can buy for your budget. alarms dont bother thieves these days whereas a lock like the one above will deter the average joe chav from even thinking abobut being able to drive away with it long before they go smashing any windows!