The front diff will need to be removed and rebuilt, or replaced with a good used unit. Is it a deal breaker? Depends on how handy you are. If you do the replacement yourself it shouldn't cost you that much (few hundred dollars), but it's a fair amount of labor. Probably not worth it if you have to pay someone to do a rebuild. Plus, as I'm guessing this is a rust-belt vehicle, rust will probably be an issue with frozen/rounded off bolts/nuts.
The V8's are great engines and can easily go 300K miles or more with regular maintenance. The V8's transmission is also pretty strong. How many miles are on the vehicle? How much rust does it have (rocker panels, dog-legs, fenders, rear spring shackles, frame)? How much is the seller asking? Personally, I wouldn't want a 16-17 year old northern vehicle due to rust issues, but that's easy for me to say (I live in the south). I paid around $2100 for our '00 AWD Mountaineer 3.5 years ago. It had 159,000 miles on it, has a tiny amount of rust and was less then aesthetically perfect inside and out, but it's turned out to be a fairly reliable vehicle and AWD doesn't bring a premium where I live.
A complete front diff rebuild can get pretty expense if you're paying someone to do it and doing a rebuild yourself isn't just a matter of replacing parts. If you don't know what you're doing you can still end up with a noisy diff even with all new parts installed.
Best way to find out if the vehicle was built with the towing package is to take the VIN and lock it up on Ford's ETIS database. It will tell you everything about the vehicle. I believe the Explorer's that came with the towing package also have an extra leaf on the rear springs (five? vs four?).