I prefer a 13 spd in a dump if that cannot be had try to find a super 10. I would not do a straight ten under 425HP it will be a dog if you have any elevation changes. 8LL are not bad but most around here with 8LL are ex cement trucks tend to have the smaller motors in them for weight savings not a good combo for a dump imo.
make sure it has a jake brake. Double frame is nice but unless your hauling boulders demo etc I would not say its not a must. If the truck is going to be a primary hauler on road most of the time I would get something with airbags. If your going to be off road alot nothing can beat a walking beam suspension for traction etc.
make sure you have locking diff on the truck no need for two unless your going to be mud bogging lol . I ve never got either of my trucks stuck and ive put them through alot.