I think a 50w heater will be fine. The temp depends on where you live, and what time of year it is, along with the lights, but in general the tank will run fairly hot. I have a 50w heater in mine set at 78, and most of the time the temp is 80-82 when i dont run a fan. However, when I leave the window open accidentally in the late afternoon or evening the temp sometimes drops to 75 or so. But your tank probably isnt by a window and won't have this problem
btw, my tank is still cycling so this temp variation isn't hurting anything. Once the cycle is over, i will try to maintain 78 degrees with the heater, light cycle, and fan