Hey Tucker,
As far as used BCS machines, you can find some pretty good deals in the older models. I'd stick with models with differential - they're easier to handle, turn shorter with steering brakes, and have better traction. I'd also avoid the biggest BCS models - 745, 945, 946, 948. They are a good machine, but are very bulky and no more durable than the smaller differential machines. In fact they used the exact same transmission as the smaller diff machines. The only difference is they put gear reduction along with taller tires and a thrid stud for PTO attachments. they're also miserable in mowing applications. If a good deal came along, I'd buy one, but wouldn't bother for an average deal.
Here's the models you should consider. The 735 did not have a reverser and is spline drive PTO. They do have reverse, but you access it through the gear shifter, not with a T-bar on the handlebars. The transmission is a little tricky due to it's straight-through pattern. You'll find you shift right through the gear you want into the next neutral. They were made from the mid-70's to the late 80's. They're a good machine with 4 working speeds in "tractor" mode, two in "pusher" mode. This is accomplished with a second hi/lo lever to operate the transmission. Fifth gear is transport - about 8 mph
The 605 and 737 were made early to mid-90's and have a reverser, but are still spline drive PTO - not a big deal, but something to note. They still have the straight-through shift pattern as the 735, but no hi/lo lever. 3 working speeds. The 605 was considered a mowing machine and had the 4th transport gear in "pusher" mode. The 737 was considered a soil working/transport machine and had transport gear in "tractor" mode. Otherwise, no difference. If you're gonna pull a trailer and want to use transport speed, you'll want a "tractor" machine.
From 1995 to 2001 or so, they made an 830 and 850. 3 working speeds and 4th transport speed is in "pusher" mode. The only difference between them is the 830 came with smaller engines and smaller tires. Otherwise they're exactly the same. I have an 830 I repowered with a Lombardini LD440 diesel - great machine.
2001 to current models include the 852 and 853. The 852 is the "pusher", and the 853 is the "tractor". Watch out for the ones with the brakes on the steering column. They're a PITA and you'll find you never use them. these machines are the best ergonomically, but you're gonna pay a quite a bit to own one, even used.
Hopefully, this gives you a little guidance when it comes to what models to seek.
Bill in WI