I'd reccomend getting a local NSI or BSIA approved firm to come out and survey your property.
ADT and most other nationals don't really give a hoot about small domestic intruder alarms. (well the salesmen don't, they just want the big projects for their commission)
Have you decided over wired or wireless installations, Wired are the better option but you will pay for labour costs and the upheaval of the property, whereas wireless (which has come on leaps and bounds, but still a little flakey sometimes) can be fitted usually cheaper due to the ease of installation, however there are ongoing costs of batteries every 2-3 years in each wireless device (most are of the CR123A battery type) and depending on manufacturer, some take 1 or 2 batteries.
Or you can go for a Hybrid solution, which means the external SAB (bell) keypad and internal sounder are usually wired and the detection devices are wireless and wired.
Honeywell Galaxy or flex series are my prefered panels, however, RISCO, Scantronic and Menvier all have good panels. Just none really beat the Honeywell panels in terms of functions or ease of use.
If you need any help on any deciding factors give me a shout.