A 19' offset will typically pull harder than a 19' conventional disc. Without a bit of background on Gypsum, it's strictly a WAG on how much power that would take relative to "dirt".
At any rate, a 19' offset working "average" soils would probably want 150+ hp. (Where as a conventional 19'er would take 110 to 125hp in dirt around here where I live)
Are you looking at new? or a good USED tractor. There are some outstanding bargains to be had on used 4wd AG tractors. again, just a guess on my part, but I'd think you're gonna want something closer to 175hp.... No harm in having too much grunt, but it would be a shame to spend the money and find you're underpowered.
19' offset is a heck of a load...