Re the buy on eBay etc concpet.. I was quoted $395 to supply and install a Tekonsha P3 or $345 for a P2, and i ended up buying the P2 from an autoelec for $155 DELIVERED and then did the rest myself. Now I get that mybe you dont feel like you want to do the job yourself, but you get the idea that the local Caravan place that quoted $345 was essentially charging me $190 to install it. when we started talking about that I didnt want it under my left knee the price started to escalate rapidly. Seems $95 an hour was the going rate!! I'm in the wrong business!!
Make you're getting a reputable brand of controller, one that you are happy with or have researched and are happy with, not just the brand the autospark has a "special" on. NOt all brake controllers are the same!!
Good Luck