I have an Adler that I use, it is an older machine but it works GREAT. There is a big difference between using a home machine and a commercial one. Mine will sew 2 thicknesses of dirty old semi-truck tie down straps no problem probably punch through 3 or 4 layers if I tried.... never needed to though! I do seat covers as well. (did the rear in my Barracuda, and all of them in my 59 Suburban). On an older commercial machine, make sure you get one with a walking foot as well as a reverse.
Harbor freight has one for $269
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=3914
table is extra I believe, $135...
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=3929
I have never looked at this one or used it though... It might be Ok or you might be better off with a used one for 5-700.