I'm surprised that the 70-hp, four-stroke won't troll down slow enough. I used to do a lot of trolling with my 50-4 Yamaha on a former boat, and it would go down to as slow as I'd want. However, that was on a different rig as well. I've done a ton of trolling for salmon, trout and walleyes and can offer my advice, which with $5, will buy a cup of coffee in New York.
If I were to pick a boat for trolling, I'd probably look at a vee-hull, and Lund makes good ones. Currently, the company is running a special pricing on a 16 or 17 footer that I would consider if I could upgrade the motor to the maximum for the hull. G3 also makes vee hulls that look good online, and trading the boat back to a G3 dealer might be a good financial move.
But as for trolling--
First, don't even think of a transom-mount electric motor for forward trolling. They are beasts to control; as one of my friends said, "It's like trying to push a rope." A 24-volt bowmount does a much better job of pulling that rope, er, boat, and it is much more efficient. However, it may not reach the speed you want against a strong wind or in current. A small, four-stroke kicker is the answer to that.
On the Columbia River, where I used to fish, walleye guys would run upstream, deploy the bowmount and troll spinners downstream. Once they reached the end of the reef or structure, they'd reverse course, fire up the kicker and pull crankbaits back upstream.
For that kind of use around here, I'd think a four or six-horse four-stroke would be ideal.
I'm thinking about doing just that as I want to do some striper fishing, and I'm thinking a kicker might be the ticket. The 50-two stroke I now have does troll fairly well, but it also uses more gas than I'd like.