I used to Tow in highschool with my truck. (4.0 OHV) you can expect around 14-16 with an ok load. i would NOT recommend a plow if you want the transmission to last. the 55re5 or whatever it is isn't worth it, Niether is the A4LD like in my truck.
if you are going to go with it, Get the best fluid your money can buy for the tranny.

 

I tow my dads 16' tandem axle trailer a few times a year for hauling mulch, topsoil, and other various landscaping duties. Does just fine. A couple yards of topsoil is VERY heavy though but I only pick it up about 2 miles from the house and drive very slow...would not want to attempt to take that out on the freeway with the Ranger! The trailer empty or with a quad or two on it is no sweat though. Having a good receiver hitch on the truck is a must have too. And like everyone else said, keep the trans fluid clean and changed and don't be going around flooring the truck with a trailer load on it. That trailer has surge brakes on it which helps too. The ranger brakes aren't really up to par when pulling trailers without brakes. Of course if you're gonna be hauling big loads all the time I'd look into a larger truck (F150, F250) As for the MPG, I never really checked with a trailer on. The truck gets driven so little (4000 miles in the last year) and gets somewhere between 16-19mpg generally but I'm pretty easy on the go pedal.

 

i tow my car behind my ranger for hours on end with no issues. total weight is somewhere around 5k i mean i can feel its there obviously but it doesnt struggle to get going or anything

 

Have '98 Ranger 4x4 w/ 4.0. New plugs, K&N, MAF cleaned, Seafoamed, etc. I get 15-16 mpg no matter how I drive it. Just a fact of life.