When choosing home security just make sure you have a reputable installer - Brinks, ADT etc (unless you actually know someone that has their own company which is fine too).
As far as wiring up an "old" (read - already built) house the process for alarming it will be slightly different - the installer may elect to use a wireless system vs a hardwired system - I have had both and had no issues with either.
The installer will cover this, but you'll want to make sure of the following:
1) multiple keypads - one in the master bedroom is a very nice especially with ALPHANUMERIC display so that you can seee what zone has been tripped, should you experience a break in while asleep at home
2) panic/ambush capability - sets off a silent alarm even though it appears that the alarm has been "turned off". This can also be augmented with a keyfob remote which helps if there is a potential fear of a "push-in" burglary attempt i.e stranger at the door - looking to push the door in when you open it - you have key fob hidden in hand or pocket to trigger immediate silent alarm.
3) coverage on all lower floor windows - upper floors at your discretion, but a carefully placed motion sensor will cover that and save you some money
4) fire/smoke detectors - usually part of the standard package
5) check local laws regarding a permit fee, and or fines for false alarms
6) exterior and interior sirens - the exterior one should be available with a flashing strobe light as well - nothing like the cops pulling up to the wrong house in the event of an emergency - lets them know exactly whose house is having an issue
7) if you have an attached garage - you may want a keypad there - this will allow you to perform a "max arm", or "instant arm", which means there is no 60 sec delay on an entry door (all other points of entry will be instant except for entry to allow you to get to your keypad and turn it off
8) also make sure that there is a bypass mode set so that you can turn off your motion sensors, for when you want to arm the alarm while staying home - (perimeter protection) pretty standard stuff.
BTW my service also runs about $20-25, month.