Usually the pitfalls are not having it installed by an installer who does it everyday, as DIY usually isn't as good , as you say you have limited experience, they can always hand it over to you working and you could possibly maintain it yourself.
What you choose will also depend on what your trying to protect , if your spending £300 then I assume nothing worth much and your not to fussed if it works or not, but £1000+ for an alarm and separately CCTV £2000+ , these are just starting points.
Your reading about dahua and hik being an overkill? Even the basic diyer is fitting some of there range , which has a cheaper end of the market and some stuff more expensive
You have a floor plan? How many doors windows , etc difficult to say anything if you don't share that , standard square building ? 5-8 cameras or more? I seen doyers spend £2k just on equipment and unlikely they will need to change anything for ten years
Need info