Give the radiator support a double check, its really common for them to be rusted out in the snow belt and they are not the easiest thing to source. I see why the truck has your attention now, the lift and tires could be worth the purchase price of the rig, an ext cab xlt has merit as well. A trans shop will charge you about $650 to R&R the E4OD, use that for some leverage on the sale. You are going to do it but your time has a value as well.
I would look at it this way, you are going to have to keep this truck and drive it to get your return out of it. If you pay 16-1800 dollars for it and put another 1200 dollars into it, looks like it could be a nice ride for 3 grand or so. You should be able to drive it for a few years and still get 24-2600 for it in the end.
The key is, it may be a good truck for YOU. That's a decision that you have to make. I am scared of body work, rather pluck engines all day long.
I'd buy a body patch panel for the lower ext cab area and cut it in. The body lines and paint would probably never look right again.
I'd probably put a flat bed on it to avoid further painting.
IF the trans in the bed is good and it will go down the road without much more than the trans R&R, I would put a 14-1800 dollar tag on it. Beings you are competent in body work and painting, this truck seems like a good deal for you.