(all below is in an American/US context)
Read about ILECs/CLECs. Understand what the cable offerings' contention model is.
Do the math.
Understand the support burdens of NAT-on-NAT.
Run the numbers of RF contention, and shared-medium bandwidth management.
Do the math.
Probably the best advice I could give here: It's very easy to do a mediocre job of all this. It's a fair bit harder to do a good job of it. And think of how much you dislike the mediocre ISPs.