If the Ron Thompsons are all you can afford go for the . They may not be as compact or as durable as some, but they are at least compatable, come with everything you need and come with a case for transit protection.
My set of alarms from another cut price company simply didnt last long enough to recommend, no names, no lawsuits. But most of the stuff people buy from them is c***.
One alarm which is British, and solidly built, is the "Good Fishing GFv" and cheap for what you get.
Do a search on that, and see the current deal,if you can stretch to the extra funds.