I have a 1999 Sabre 16/46 hydro. It's my second mower that I keep at the mother in law's to mow her lawn. All in all it's been a good mower. I had to replace the deck at a tune of $550 3 or so years ago. It has a 16 hp Vanguard engine that is starting to use a little oil between mowings. Starts every time thou. It has the K51 Hydrogear trans axle. Years ago I changed the oil to Mobile !. Used Mobile ! in the Vanguard after the break in. The bearings in the spindles need changing. I enjoy the mower still. Sabres are old mowers. I think 2000 was the last production year. The JD 100 series took over. I can get most parts at the JD dealership. I would never spend very much money on it form here on.
What to look for? The deck top and bottom. Make sure it's not rusted. The spindle bearings. Listen for grinding noise, and play in the shaft by holding a blade tip and pushing up and down, back and forth. Look for oil leaks on engine and trans. Look for steering play in the joints at the wheels. Tire side walls for dry rotting. Paint on deck and frame. Battery area for rust and rot. Does it start easy with out smoking. Does it run smooth at wide open throttle with the blades engaged? How much of the blades are left? Check the air filter. Is it used and clean? Is it new hiding a problem?
I could go on and on. It's an old mower. Repairs and parts cost money. Some more than the mower is worth.