Half a day for cabling, half a day to mount and connect the detectors, panel, ancillaries and program/commission/handover... £150 sounds about right. When you do it for a living, cabling can be quick!
I'm not sure that you've understood the beam - inside your Optex device there will be (say) some high power infrared LEDs and lenses that "shine out" of the case through the plastic. Somewhere else, there will be an identical-looking unit that contains IR transistors (receivers) and lenses. The internal lenses are "aimed" at each other so that the beam(s) of IR light from the transmitter go directly to the receiver. Physically, this could be straight out, away from the wall, or sideways, along the wall. Anything breaking the beam will cause the receiver to trigger its alarm relay and this is connected to your alarm... If you were Tom Cruise, you could get special glasses or an aerosol can and see the beams (but not in real life!!).
The GJD device is a PIR - PASSIVE Infra Red detector. This doesn't emit anything - it only looks. PIRs are fitted with with many lenses that focus the IR emitted by warm objects (generally, living things) onto a single sensor. These lenses create "looking lines" that are often (wrongly) referred to as "beams". They are NOT beams as the device is passive. When a warm object passes across one of these looking lines, the IR creates a "flash", and therefore an electrical pulse, on the sensor. The PIR's electronics counts these pulses as the object crosses the looking lines and triggers its alarm relay at the appropriate count.
Use the active beams (Optex - other makes are available) as an invisible barrier for perimeter detection. Use a PIR (GJD) to detect people (or animals) in an area.
You can use these devices with (virtually) any make of alarm. The only time that you need to stick to a particular manufacture's detectors is if you are using wireless, as the radio signals are all proprietary (another good reason for not using wireless!!).
Finally, one thing that may be worth considering is fitting smoke detectors to your alarm. If you already have interlinked, mains-powered smoke alarms then don't bother. Having some smoke and/or heat detectors connected to your alarm will mean that you can use the alarm's comms to tell someone that they've gone off. It also means that your alarm company will service them and you won't get woken at 4am by the annoying be-beep of a dying battery! Menvier M12 is good as is the Texecom OH-blah-blah (sorry - can't remember the part no.).
