as an electrical engineer and security engineer this is my advice to you,
The first thing you should do is enquire if your insurance company require the following;
1) NACOSS (insurance backed company)approval for your system
2)Audible only (loud noise and nothing else)
3) monitored system, alarm call from control panel to Alarm Receiving Centre then to the local plod and or fire service.
If you just need to install a new alarm then i personally (from over 7 years use) reccommend the Gardtech 816 panel with a Contour keypad, the panel is so easy to wire and so easy to program, and if you need it approved there are a lot of companies out there that will adopt the system and then get it approved for you.
Basic system layout, abbridged version of BS4737 (soon to be removed and an EN code to replace it).
all doors leading out of the property to be protected by proximety switches (magnetic reeed contacts) prefered that each one goes back to the panel, (but you can link em all together if you want).
magnetic contact to final exit/entry door
panel must be in a secure protected area, all we do is put a PIR near it.
keypad must not be visable to the outside world.
PIR / Dual Technology sensors to be positioned to ensure correct trap protection within the property.
Or you could do what most people do in your case, call three local companies, (keep away from people like ADT and CHUBB, they are only out for you hard earned) tell em what you want and let quote you a price.
If you need any further help let me know.