The Troy-Built has timed blades, and when the timing belt snaps there is almost always damage to the blade spindles, pulleys (which are plastic), and / or deck pulley brackets - which are made of very thin cheap steel that bend easily.
See if he will sell you the Proline only. That Troy-Built could cost you up to $400 in parts to fix and you would be lucky to sell it (fixed) for that price. The seller probably knows this which is why he wants to sell it with the Toro.
Good luck either way!