OK, I'm gonna give a stab at your question....
The tranny is among the most desirable of the bunch. I don't have one myself, but have done extensive reading in the forums.
CAT engines have a mixed reputation - the 3126 was available in different configurations, from 170hp to 330hp.
https://en.wikipedia.org/wiki/Caterpillar_3126 Your engine horsepower is probably printed on the dataplate.
My Cummins 5.9 is rated at 215 hp, but is better known for reliability and fuel economy than brute force. That's where my priorities lie, so it works for me.
At least as important as the two items above is how the rear end is geared.
As far as price is concerned, I would guess (with a blindfold on, sight unseen) $3,500 upward - depending on whether you are buying from an auction, or dealer, or private sale.
As far as conversion space, options are up to you. There are a myriad of builds posted here. We took a home design program, entered our dimensions and went from there. Everyone has different needs/priorities/wants, and no two are the same. Lots of things I swore I didn't want when I started have become priorities in my build after reading about other's builds.
Not sure how big (heavy) your "pretty good sized" trailer is, and where/how fast you plan on going (mountains at highway speeds), but that may be an issue depending on what your expectations are. I don't do mountains (and if I decide to, I will plan on being in the slow lane with the loaded semi's), but I do plan on towing a small car - at least in the flatlands.
Good luck with your project - lots of info on these forums if you're willing to read and get up to speed.
Hey it was good talking to myself!!
