A good flatbed/step/heavy set up?

Most engines will do the job just fine, I would recommend staying in the late 90's early 2000's era. Series 60 Detroit, N14, or 3406E/C15. 18 speed with either 3.70 or 3.90 gears. @1400 rpm you will be running ~62 with 3.90 and ~65 with 3.70. But with an 18 you will still have plenty of low gears to get you moving when heavy or if you are in soft ground when at a jobsite.

 

You can still get the series 60 Detroit without EGR and DPF (basically, no emissions control) from Western Star in the 4900.

Was scopping out their reviews last night on youtube, guys seem to like it. Parts are easy to find and a lot of mechs are well trained on it.

Edit- Sorry, just reread the brochure, it's only available on vocational vehicles.

 

I am kinda partial to the 1997-2002 KW T800 with the 450 HP N14+ and 13 Speed. Running 24.5 tires, you should be able to avg 6.2-6.8 most days and have no problem pulling whatever you want. From the factory, the 3 Axle has a GVW of 85k. You can either add a Pusher (Hi $$$) or find one with a factory 4th. the N14 makes good power, has a great Jake, and costs less to rebuild that a Cat.

 

Depends, For most Flat/Step work if you can scale 46k lbs of freight, you will be just fine. If you are worried about weight, look at your trailer super close.
When you say Heavy Haul, how heavy are you looking to go?

 

4 axle T800 500hp Cummins and 18 speed.

 

4 axle truck and 4 axle trailer gives you 150k lbs. Go with a 3 axle trailer with removeable stinger for more flexibility.