Diesel or gas: Easy. Like everyone says, diesel.
Which flavour truck? Mostly flip a coin I guess. My bone stock 2006 Isuzu powered Chevy LT has been flawless, besides routine wear parts, for 200,000 miles. By far the majority of those miles it has had a 12,000# 5er on the back of it.
2WD or 4WD: That depends. If it is going to be used in snow or offroad with any frequency then 4WD. If strictly a fair weather truck to pull the trailer then 2WD is just fine but carry a tow strap just in case you do need some help. Ours is 2WD & only need a tow in soft ground a very few times.
SRW or DRW? An option not mentioned but a very important one. The added stability of DRW cannot be overemphasised. Ask anyone with a heavy trailer who has gone from SRW to DRW. The often cited argument that a DRW is too wide in the carparks does not wash. Park further from the door. Get out & walk. You need the exercise.