Nothing wrong with buying used equipment, there are some good deals to be had. What you want to look for in a used mower is the number of hours on the equipment and the maintenance intervals. I have a commercial gravely mower that I bought with about 950 hours on it and about 15 yrs old. the manufacturer recommend a major overhaul at about 2000 hours. like you I have about 3 acres to mow and it takes about 3 hours each cut. So if you do the math at 3 hours a cut and I average 28 cuts a year, so 3x28=84 hours a year. now if I need an overhaul at 2000 hours and there are already 950 hours on the mower, that means I have 1050 hours to mow until I need a rebuild. So 1050hrs / 84hrs per year =12.5. years of use until that overhaul window. It's a good general guide to help you figure out the overall value of your purchase. Good luck!
P.S. I would not go smaller than a 60" deck - and if you have uneven ground the independent front suspension is a nice upgrade.