Truck comparisons

I'm currently looking for a truck. I need something
that is capable of hauling light loads (i.e. A horse or
two, small firewood loads, etc), halfway decent gas
mileage (14-20 ish) for everyday driving, and not
going to require a ton of maintenance right off the
bat.
I'm generally a dodge fan but really I just need
something reliable. So my questions are pretty
general:
Looking at used trucks, under 100,000 miles good,
bad? Is it gonna start needing parts at that point? I
know gas and Diesel engines age differently as well
as manual vs automatic; but I'm not sure which
generally last longer.
Any information would be helpful and appreciated:)
Thanks!