One more thing, if you decide to go new and buy a gasser, find/order you a 4.30 geared truck. Remember these trucks have a double over drive with a .67 final ratio and a motor that build power high in the rpms. There are lots of threads on this, and I'm not going to start another 3.73 vs 4.30 gear war here, just do your research. There are 6.2 and 6.7 sections here that you can learn a lot from.

 

Other than the Re-Gen light coming on the dash I'd never know there was anything going on. It's a seamless operation on my 16' F350.

 

DPF is the soot cooker (diesel particulate filter)
DEF is the diesel exhaust fluid the stuff you have to put in for the NOx catalytic converter

 

Now Ford has decided it doesn't like any new oil for its diesel engine so people are hoarding remaining stocks of the old stuff being phased out.
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For me the diesel/ gas thing can be summed up like this:

Repairs:

Gas - Few hundred bucks
Diesel- Few thousand bucks

 

OP - if you're looking at gooseneck/5th wheels, then you'll do well to go 3/4 or 1t (the 250 and 350 srw are basically the same truck, but the 350 has a higher legal weight rating).

Gas or diesel really is a personal preference - My personal truck is a '15 350 srw 6.7; my last work truck was a 250 gasser... I pull an 8800 lbs camper with my 350, and pulled 8000# generators with the 250; so pretty much a head to head comparison.

Either will do what you want just fine.

The 6.2 revs high, and drinks a lot of gas, IMO - that's why I went diesel for my personal truck. The routine maintenance is more on the diesel, but you do it less often; so for my usage, anyways - it was about even. You pay a premium up front for the diesel engine, but you also get a premium when you trade/sell, so again, a wash...

I don't think either is a bad option - you're not towing heavy or often, from the sounds of things, both power plants have matured, and are very reliable, IMO. If you find the truck you like, with either engine, I think you'll be more then happy...

All that said - the torque from the 6.7 is pretty spectacular. I smile every time I put my boot into it, and let the truck run...

 

You might at least look at a newer half ton. The towing capicity will be much higher on the 2011 and newer trucks than your 06 and they privide a much better ride empty than any 3/4 or 1 ton. I pull a 16 ft steel featherlight gooseck stock trailer with a 2011 ecoboost and it does just fine with 7 cows in it that run 1200-1400.