While it has been some time since i have manged and setup enterprise equipment, does not mean that just because I have a home network, and not some huge cooperate office to manage, dont mean that there is any issue with using hardware designed for big and better things in the home. If for anything, just to gain experience in it.
I would have bought the ER-12 already and given it a try, but it is not for sale locally, or on Amazon (by Amazon). Only by Ubiquiti direct and they have a 15% restock fee and have to pay return shipping for a Return. So just buying one to try is a good risk there. And I wont buy it until I have enough confidence it is worth that risk. I have read that even the USG with its 2 core 500mhz cpu can do 1gig WAN fine, for most. But its a mixed bag.
Just because you don't know what you want does not mean it is a bad idea.
Your questions and answers are not of someone that knows networking.