The Juki 555 from my understanding is a fine straight stitch only machine for light to medium weight fabrics, but there are other machines out there that you may want to consider when making backpacking gear. Personally I own several commercial sewing machines and I find my Pfaff 138 ss/zz and my Consew 206RB walking foot to be a good combination. The Pfaff works great for most light to medium weight fabrics like silnylon through medium weight cordura, and I do use the zigzag function a fair bit. However, once the layers get thick, I move up to my Consew walking foot with V69-V92 thread to tackle such jobs as shoulder straps, grosgrain bindings and hip belts.
Anyways, getting back to the Juki, it would work nicely for sewing many things and the price sounds about right. I bought my all commercial machines used for under $350 complete, but they did require cleaning, oiling and timing. One thing to remember is that most commercial machines are set up for high speed production work; therefore, you will want to swap out the old motor pulley for a smaller 2 1/4" one. Luckily this is very easy to accomplish in a few minutes time, and then all you will have to do is get a shorter belt. Another option you may want to consider is to purchase a new servo motor, which will allow you to control the sewing speed more easily and precisely than a clutch motor. I have a Reliable Sewquiet 4000 which I can recommend.