As a L7, I would not even bother shelling out £100 on RefsCall's, (let alone the £500 on a set of Ervocoms!) they aren't the best and frankly outside of your charity games you won't use them. Now being a 5, even I don't have buzzers and wouldn't consider them until I am atleast L4, maybe higher.
Also, as a 7, having buzzers would make you lazy. Certainly always feeling for a buzz and not having to look will make you a bad referee and not neccessarily a better referee. Infact, a better referee would survive without buzzers and just use normal flags because it means they have to look over at their assistant, something which you alluded to in the OP. If you miss a flag, you shouldn't think about resorting to buzzers to solve the problem, you should lift your head when running with assistants and actually actively look for a flag, not meaning to sound harsh but that is the only way you can improve and buzzers certainly won't help you to do that.