I love this, good for you ole girl.
Personally I am a Ford guy, the 450-550 with a 7.3 are incredible trucks, I just rid of a 2000 550 7.3 2wd ( Detroit Locker and 3.91 gears, which i removed before the sale) and got a V10 550 4x4 for local stuff, and also recently upgraded my F-150 extended to a 2002 F350 CC 7.3 4x4 for family tripping and over the road deliveries. All older 550's have 488's (gears) and they tach the engine highly, if over the road consider regearing. An F-450 has a Dana 80 (same as F-350) and can be regeared for half the money of a 550. The shop that regeared and put the Detroit in my 550 (Dana S-135) charged $2500. F-450's came with higher gearing , 410's and the like. Also, worth considering is your GVW, if you go F-450 it will give the ability to haul a 10,000 GVW trailer without going over the "magical" 26,000 lb CDL threshhold. I dunno what your horse trailer is , but this is worth considering. F-450's usually are 15,000 GVW or there about.
You should look in Truckpaper.com and also LarryStiger trucks in Tenn or Kentucky (cant remember which) he gets lots of good fleet units. I would stay away from all newer Fords (diesels) and even the Duramax has head gasket issues from 150-200K. And Dodge, well not my cup o tea, but the Cummins is great, to bad about the rest of the truck.
Look to getting a plane ticket one way to pickup your dream truck from down south somewhere, LOTS of "units" come out of Texas etc....I have seen some really nice clean older trucks with reasonable mileages, yes the 7.3 will cost you a premium, but the reason is it's longevity and durability (all good reasons). IF you go automatic, the earlier Ford auto needs a $3500 rebuild/ replacement (go Jasper as nationwide warrentee) every 150K, which in the same mileage you may replace the clutch twice on a standard which ends up cumulatively costing you the same with out the hassle of shifting all the time.
It is highly likely you will NEVER see the mileage per gallon of the older diesels on the new ones due to EPA regs and on truck systems (urea etc..)
FYI, F-350's that came "Cab Chassis" have a straight frame like the 450-550 but are slightly thinner (1/4 in stead of 3/8")
I will PM you.
Enjoy your adventure.
Jronwood