Giving us the full and complete details on what truck you're looking at may help with suggestions since there is always "something" to look out for.
Other than that and in regards your questions.....
Yes, that is the HO motor. But dont get hung up on such small technicalities either because the SO motor is sometimes the desired motor for power adding reasons. And good news is you're not looking at an auto tranny truck since the manual tranny trucks are essentially a SRW 1 ton truck.
And 3000 lbs in the bed is going to be a lot for most all trucks. Even for a DRW truck. And in saying that, the 3rd gen trucks have softer suspension which tend to not carry as heavy of loads stable as the 2nd gen trucks. But..... 3000 lbs will most likely justify the need for some sort of suspension help even with the factory overloads. That being in the form of airbags or Timbrens. I like my Timbrens and they're only a couple hundred dollars.
Do I think the 245 hp Cummins can handle 3000 lbs going up hill????? That 6 speed manual tranny Cummins could "idle" up that hill without breaking a sweat. Thats being totally serious too as these engines actually produce 300 foot lbs of torque at idle, so.....yes it could truly idle up that hill. Seriously though, I'm not sure any truck couldn't handle driving up hill with 3000 lbs in the bed. But if you ever feel like dragging 20k + lbs up that hill and then you would definitely NEED a Cummins.
Are there any straightforward mods? Yes, you can add a mild timing box like the Edge EZ which will add 60 hp / 180 lb torque.....to the ground. To put that "mild" power increase in perspective, most gasoline engine chips add around 1/4 that to the crank. This is absolutely why diesels have become increasingly popular with the hotrod crowds.
Cummins is well known for reputation for reliability too. The 2nd gen trucks are still very desired because the cheap and easy power you can crank out of them is something the newer trucks cant offer relative to the costs and ease. Plus with emissions and diesel smog laws bearing down on the diesel community faster than we'd hope, the 2nd gen 24 valve trucks are pretty much legally exempt from anything you change or add except for timing/fueling boxes (which are easily removed before inspection) or overly larger injectors which would make them smoke too much during inspection.
The only reliability issues the 2nd gens have is the fuel system. Aftermarket fuel pumps and usually replacement fuel injection pumps are in order at some time or another. Still though, both of those items are cheaper and easy than that of similar items on the newer trucks. Again, this is why the 2nd gens are popular.
But all that said, $15k is not cheap so you must be looking at a really nice truck. :thumbsup: