Welcome to the forum! Almost certainly, for $800 you can buy it and fix it for less than it would cost you to buy one in good shape. The question is how high a price you put on your labor. If it's just a matter of draining and replacing the fuel, swapping the filter, and bleeding the system to make it run, that's a terrific deal if the rest of the machine is in decent shape.
Other stuff to consider, obviously, is your area and street prices on comparable machines in similar condition, that run fine. Are you planning to fix it up then sell it for a profit, or do you need it to do some work around the property?
Personally I'd do it, but I've gotten a kick lately out of finding project tractors like this, and getting them up and running again, just as a recreational thing. Your interests and needs may vary. Good luck, let us know what you do and post some pictures!