Let me ask you this...Does it need to be 4x4? Stick with me here. You said your boy likes fishing and he's a outdoors kind of kid. Does he go places that a 4x4 is necessary? I have owned many 4x4s in my life, and can honestly say I have probably locked up the front ends 20 times. Mostly because of my dumba$$. Most of the places I went, I could have gotten in ,and out...and did with just 2WD. Maybe a nice 2WD would do just fine with a set of 31s.
I know having 4x4 is like having a gun. It's better to have it, and not need it, than need it, and not have it.
Can you get some pics? I just feel like this truck is going to cost you more than you know. Fixing someone else screw up can get quite expensive, and makes you second guess your decision.
As far as the 10-12 MPH...that's not great, but than again these trucks never got great gas mileage. When I had my 77 Jimmy 4 inch lift with 3/4 axles...4:10s. and a 400HP 396, TH400 I was getting 12-14 MPH. and I drive fast.