I've got two Howse bushhogs, a 4' that we have had since 1999 (using in conjunction with my Yanmar F16D) and a 6' that I've had for about a year (used with my Kioti DK45). I've had no problems with either one of them. The 4' model has been a very good machine and has received a couple hundred hours of usage, both for cutting grass and pretty rough cutting (about as rough as you want to get with a little tractor, anyway). Sadly, the bushhogs set outside most of the time, even through winter, and the neither has shown any sign of deterioration. The 4', even at over 4 years old now, could be washed off and touched up and would look almost new, with the exception of several dents and dings. The Howse line in general is probably not the best product in the world, but it is (in my experience) a great product for the money.
Still, since a new Howse in that size can be had for $400-$500, I wouldn't go over $200-$250, depending upon condition. After all, it is used and you don't know it's history. Good luck.