James, I just picked up a pfaff 130 ( i took a risk and found a decent deal on ebay) specifically for pack building with the understanding that I will probably spend another 50-80 getting it serviced.
Madeline, everything I've read and heard about the pfaff 130 says that it is one mean workhorse and one of the few affordable home machines with enough power to do heavy material sewing. It's a favorite among sailers for sail and boat upholstery repair. The general consensus is if it fit's under the pfaff 130's presser foot, it will sew through it.
Other people will know better than me (and there are far more experienced sewers on this board, I'd wait for them to chime in), but I don't really think you can get a home machine that will do both heavy duty work and wispy material work very well. If anything, it'll be mediocre at both. But I'm pretty sure silnylon is thicke enough to be fine w/ the pfaff.
For very wispy materials (7d, 10d) I love my singer 503a. The 500a and 503a are the best home machines singer ever built (their own words), and are the descendants of the equally good 401 and 403. For these wispy fabrics, If you get the right needles, thread, (e.g., Schmetz Microtex Needles 8/60 and mettler metrosene plus) and get a straight stitch needle throat plate (instead of the typical zig-zag plate with the larger opening), you should be all set.
My first machine was a cheap newer Brother machine I picked up new from amazon. It does okay, but even when new, it always sounded like it was going to implode any minute. It also could barely go through webbing. I'm looking to get rid of it, but it's the only free arm machine I have right now.