I have a model 5418 commercial walk behind. 18 hp dual cylinder B&S, 54" deck. I bought it new in 1997. has almost 300 hours now. It has been a great mower. I cut my 1.25 acres and mow semi-commercially (a few properties per week). I have no problems with the mower and have bought parts at the local dealer over the years. A buddy of mine bought the "TANK" last year and loves it. I understand that it is a highly desired mower in the comercial mowing community. For reference I bought mine for ~$3500, the TANK lists ~$7400 for a 60", 25hp model.
Hope this helps.