I'm going to throw my tuppence worth in, even though I'm nowhere as good as the others posting here, but having installed a couple of accenta and scantronic alarms, if you're going to hard wire the bell - which I found the trickiest part - then you might as well go for broke, and do the sensors as well. Now I know someone is going to critisise the Accenta as being a basic alarm, but if you fit vibration sensors on the windows and doors, it does the job of being a deterrent, and the alarm will go off before the idiots have got inside
Alarm peripherals are fairly straight forward in design, so if you run 8 core cable around the skirting boards, then you've got 2 cores to power the item, 2 cores for tamper, 2 for an alarm condition, and a couple for spare. The tamper circuit needs to be set up as a compete loop, so you need a connector block to link all the tamper circuits together. You can have 2 keypads (normal one downstairs, and a second one upstairs for setting the alarm at night.