Firstly make sure you can patch radio, intercom and headsets together - this is sometimes less straightforward than you'd think. If you haven't already got headsets and an intercom for the aeroplane, your best bet is to buy a complete microlight / homebuilt package from somebody like Communica or Lynx. Standard light aircraft headsets aren't always very good in a noisy homebuilt or microlight cockpit.
The prices are pretty standard, but they don't as a rule wear out so look around for second hand units. Some companies (such as Harry Mendellson) can save you a few quid compared to the standard Transair price.
You obviously need an RT Operators license, which most people get with their PPL, if not ask at your local club about courses. You also need an installation license (in-fact a transportable radio license) which costs about £30 and simply requires you to fill in a form from the Radio Communications Agency.
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