New is new. Is that used one coming with a warranty? Why is the guy getting rid of it? Only $500 difference? Again, go new Raptor SD 54".
How many acres do you have? I mow 3, sometimes 5... they never mow what they say but still if you have one acre it shouldn't take you too much longer than an hour with lots of trees. My front yard is over 1 acre and can be done in 45 minutes (1.25 gallons) going the short way (more turns). I can do the rear 1.5-2 acres in 50 minutes going the faster long ways (1 gallon and buildings).
Now - I do wish it had a larger tank and went faster... I haven't even touched my back 3 acres. Kinda wish i got a 60".
Then again, I'd put the $1000 in the old one first.