The best way to save money on insurance is not to claim! By installing security systems you demonstrate to the insurer that you are less likely to claim and Thatcham immobilisors and Alarms are amongst the best. Also if you display a sticker advertising the fact that you have this level of security fitted to your vehicle, then you are less likely to suffer fro an attempted theft or break in.
However, sometimes the cost of this security can outweigh the savings available, particularly if you intend to keep the vehicle for a short time period.
The best way to find out the saving that can be made , is to search for insurance both with and without the security installed and then see what the difference is. You can multiply this difference by the number of years that you intend to keep the vehicle and then you have the budget to spend on security!
To find out out security features, I would recommend Halfords.