How much trolling motor on my 12' Semi V?

So I'm finishing up the paint/steelflex on the boat, and then I will be ready to install the short deck that I'll be mounting sonar and the trolling motor onto the bow with.

How much trolling motor do you think will be needed to move around a 12' semi v with floors, a 9.9hp on the back, batteries, and maybe 2 guys?

Also, there will be times I'm on a electric only lake with no gas motor on the back and will want enough to move the boat around even if I get into wind.

Right now I really want a Minn Kota Edge, and Wal-Mart has some awesome deals on the 40lb - 42" model, as well as the 50lb - 42" model. Do I really need 50lb of thrust, or should I save the $$$ and get the 40lb model?

Personally, I'd go with the 50#. The old saying "go with as much hp as your wallet will handle" holds true. I have a MG 54# on the front of my 16' Lowe Roughneck (fairly heavy), and it gets me around the open lakes pretty good, even when the wind picks up a bit. On the rear I have a MK Endura 40, and I wish I would have bought one with more thrust. It doesn't fair too well in the wind on the open lake.

40 will probably be fine, but I would go with 50. Better to have more power when you need it.