Well done for not going down the Yale tat route, the Texecom 24 is fine but pay careful attention to your pirs and dts.
I would recommend a healthy budget for these, consider texe brand by all means but also risco iwise and vanderbilt e-line.
Dont just buy the £4 budget ones.
If your alarm is bells only you are essentially wanting to avoid ever having a single false alarm, as your neighbours may come over the first time it goes off, but regular false alarms will erode confidence in the system.
That said, system design and sensor placement is a huge part of why a professionally installed system is worth the extra expense, as well as the ability to have proper monitoring.