Any used battery purchase is taking a chance. For that matter, any 12 volt "deep cycle" is taking a chance. If it was actually recharged after each use it should be fine to start with. If you make a mistake and ruin your first battery, a common problem, the learning tuition is only $40.
It is my opinion based on my experience that regular starter batteries can be cycled to a 5% depth of discharge, occasionally 10%, without reducing the battery life. The 50% recommendation is for golf cart batteries. I limit discharge of my 75 amp hour 12 volt battery to 10% regularly and rarely twice that. For a MaxxFan, some leds and a phone charge that's not a problem. A 100 watt panel is good to recharge that.