A professional alarm company shouldn't offer you a system you can buy from Cost Co. B&Q and Screwfix. You should look for brands like Pyronix, Texecom, Orisec, Risco and other alarm manufacturers.
Orisec and Texecom are made in the UK, Pyronix is currently made in the UK but is now part of Hikvision, known for the value CCTV brand.
Look up on forums about Yale alarms and Swann CCTV, think you may find it useful.
If going wireless, you need to think about the alarm panel location and the location of the wireless devices. Avoid large metal objects within 1M of a device where possible
Also to consider is frequency hoping, this is where a wireless system can use a different frequency to communicate with the panel if a frequency is jammed, Pyronix, Orisec and Visonic have wireless systems that can do this and you will have to check the equipment on Pyronix and Visonic panels to make sure its a frequency hop system.
As for required sensors, I personally don't recommend pet pirs wireless or otherwise as the way they work is pretty much blocking anything out under 4ft tall, its more complicated than that for some sensors. However consider a cat climbing above 1.2M(4ft) the PIR will trigger, consider a person under 1.2M the alarm may not trigger. Often kits contain petwise sensors and I and others are regularly trying to stop manufacturers from doing this.
Otherwise I would recommend QD(quad element) PIR's for pretty much everywhere except conservatories and garages where I would recommend a DT (dual technology).
Door contacts and shocks, on some systems its possible to have one device split as a door contact om one zone and a shock sensor on another, this can be useful to create a pre warning that someone is trying to break in before the door is opened.
This does depend on the manufacturers devices available and how thy can be programmed.
CCTV
NVR/DVR, seems madness to take it out and leave the cameras probably came as a cheap kit if its unbranded. In this case I would always recommend you ask the vendor for the currently details of the DVR/NVR and or cameras installed so you can find a suitable replacement.
Not all CCTV is the same, so without knowing what you have, can't advise on compatibility.
Things you need to consider:-
1. How are the camera's connected BNC, BNC plus power, Cat5E.
2. How many channels are connected
3. camera resolution.
4.Technology
5.Manufacturer
For example a Hikvision Turbo DVR will accept 2MP on version1, on Turbo 4 the DVR could accept 5MP or 8MP depending on the DVR model.
Some analogue DVR's can add IP cameras over the home network, depending on the DVR will depend if you can add any, just 2 or multiple units, the resolution capability may differ.
There are several analogue technologies out there, using bnc connectors, if the DVR can't handle the technology of your camera it will not work properly if at all.
IP cameras, use many different protocols, so may need to look at using the ONVIF protocols if the DVR/NVR isn't from the same manufacturer as the camera.
Using ONVIF, can lose some of the cameras features compared to using the manufacturers protocol.
If you have shotgun cable, bnc and power, then you may need a separate power supply to power the cameras, if just BNC connection then the system may be POC and you would need a DVR with POC outlet or a separate power supply and a balun to inject power into the coax cable.
As you can see the CCTV options are numerous so it is complicated selecting the correct DVR without knowing exactly what you have, so the best place to start is to find out what vendor was using for the DVR/NVR to start with including manufacturer and model.
