TD5 is great for towing however you need to consider several things.
The gearing on a defender is lower than a disco, (disco has 1.22 t-box and defender has 1.4) this may be good or bad depending on the weight/distance/roads you travel.
A 110 is probably more suited to heavy towing as its a more stable platform than a 90, although it will do a good job i'd be inclinded to conisder the longer wheelbase for better stability.
Heaters are not particually great on TD5's, they work but its still a landrover heater. Seats are comfortable and heated ones are great, soundproofing isnt super but i did 20K/yr on motorways and 6K offroad in mine every year and it was good and comfortable. People complain about lack of elbow room and i have no idea what the problem is, i'm 6'1" with long arms and never had a problem!
8K - small budget, you can get one but itll be an earlyish one so watch out for the usual problems with oil in loom and such likes but generally they are very reliable engines.
Have you considered a TD5 disco instead?