I would pass if you intend on using it for a DD. I agree with what others have said about using a car for a DD and having a truck parked for use as a truck.
If the biggest thing you regularly tow is just two ATVs on a utility trailer, I would just keep the Ranger. Depending on your ATVs and trailer we're looking at what 2500 pounds tops? I'm guessing you have a 4.0, so you should have no problem. If you're worried about the trans throw another cooler on it. My dad's 4.0 Exploder's 5R55E is limping now at 170K, but this is after constant abuse like pulling 4500+ pounds at highway speeds with OD on. Without an aux cooler.
Something I always consider as well is for a 2000 Ranger you can probably find parts on the side of the road. Junkyards have tons of compatible parts. I doubt you can say the same about the 250. Even if you blow the trans, you can get a new one for your ranger (not cheap, but still available).
Salt eats everything, no matter what it is. All you can do is keep washing, spraying, and wire-wheeling. One winter turned the bottom of my pristine, painted truck frame to rust city. I only wash it every six months though...