I don't have an alamo flail but I do have the ford 917 flail. Some things to look for would be the bearings on both ends of the cutting shaft and the end of drive shaft where it hooks up to the belt drive. I needed to replace all these bearings (used flailmaster parts, found on the web). I would also ask them to hook it up to a three point to allow you to run it. Check for excessive vibration and/or noise. With the mower lifted, PTO and tractor off, look at the blades are a lot of them broken or missing, more replacement parts. If you have a chance to demo it cut some thicker grass, does the belt start slipping quickly (poor or no cutting). Again parts that can be replaced, fixed, or adjusted but it is good to know what you are getting yourself into.
By the way, for my property the flail works great (6 acres of horse pasture).