What I understand so far:
- LT tires are better than ST b/c they tend to last longer and tolerate highway speeds better.
- Age (not mileage) defines the replacement interval on ST tires
- ST tires don't tolerate speeds over 65 well (heat)\
Not true. ST (special trailer) tires are designed to absorb side scrub that comes from having axles close together. Buy a trailer tire, which ever brand you prefer, run them a couple of years (depending on how far you tow) then replace them with a new set. You will find that you can get a lot of your money back by selling the still good set of trailer tires on craigslist to one of the many lawn care companies in your town.
The ST205/75R15 trailer tires are available in load range C (six ply rated) and load range D (eight ply rated)
ST225/75R15 Trailer tires are available in load range D (eight ply rated) and E (ten ply rated)
If you have the room, go for the 225's as they will easily carry the load you are talking about. Keep your air pressures up, and watch out for curbs!