First of all, Sprinters are NOT front wheel drive. Never have been in North America.
Sprinters have a 5k towing limit. If you tow anything with over say, 2500lb weight, you will need supplemental brakes on the trailer, (don't know if horse trailers have them), as the long wheelbase Sprinters are iffy as they are on their brake capacity.
If you are talking about this Sprinter as a class B motorhome, I think you'll be too heavy overall, or right at the weight limit of the combined vehicles.
The 5spd NAG-1 transmission is marginal in your intended application also. You'd need an external trans cooler setup for sure. That's my opinion...
Having owned 2 of these vehicles, I wouldn't do it