When it comes to condition, never take the word of the Seller.
A running 448 with a deck in average condition is worth about $600.00. Tires are always decent because the rears never wear out. Three point hitch is an option.
The big issue for you is the true condition of the Onan. You should do a compression test at the very least and if possible, a leak down test.
The 2nd issue is the PTO clutch that drives the mower deck. If it is slipping or does not stay engaged, then it may need rebuilding.
A complete overhaul of the Onan could run $1800.00. A new PTO clutch currently costs around $600.00. All too often, owners run these tractors but never maintain them properly. Then they sell them to someone else. So be very careful.