Personally, i wouldn't buy any alarm off eBay.

Save your penny's and buy a category 4 Auto watch alarm, that can be integrated with your old system, retaining ur keypad to start up the car. If u get a cat.4 alarm, you also get a discount on your insurance.

Alarm's are something I reckon people shouldn't stinge out on, if your going to do it, do it right, coz it can save your car/possessions.