I mow 1 acre of grass with a 52" honda commercial mower. I just repowered with a 2 cylinder 18hp honda. I can cut my yard in about 45 minutes. I also have a 36" bunton which takes about 70 minutes.
I would suggest a zero-turn hydrostaticly driven walk behind (make of your choice). 48-52 would be the best size in my opinion, zero turn so that you don't chew-up your lawn as you turn.
In my opinion, all the new ride-on tractors are kind of junk (please don't flame me). There are nice tractors from the 1960's that are really well made, so if you prefer a tractor, I would buy a beautifull old one in great shape. But, without doubt, the commercial walk-behinds cut the best. Your lawn will look much nicer if you buy a walk-behind.
Hope this helps, just my $0.02

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1992 500SEL 25K
1995 E320 40K
1995 SL600 120K
2002 JX8 Sport 43K
2005 Volvo S40 95K
2006 Isuzu NPR 304K (Frankencamper)
2007 Crown Victoria 150K
2014 Smart Electric 20K