Yep, I can imagine how grit would tear up a machine. A few weeks ago I pressure washed and scrubbed both sides of the paulin, folded it and put it in the shop. It's about as clean as I can make it. But it's also one of the camo'd covers - no idea if it's CARC or latex, or? But when I take the stuff in I'm sure they'll have some idea of the issues once they lay hands on it.
As to design and fitting, if I do go down the DIY side, I see that some threads suggest using muslin for the initial shaping? Is that accurate?
And another question - what kind of thread works for doing these covers, and what do I need to use that thread? Heavy duty needles?
Yet another question - thinking about going with a used sacrificial machine to DIY it, are any models or brands better than others? Or pretty much all the same for the low end stuff?
For patterning muslin would be fine. I've also used lightweight packing paper, like Amazon or others sometimes stuff into boxes. If you want to get fancy, Sailrite sells a patterning vinyl that is slightly tacky so it can be positioned and folded and so on. You should be fine with any lightweight material for a one-off.
For thread I recommend one of two choices, either Coats and Clark OUtdoor upholstery thread, which is at well-equipped sewing/hobby stores, or if you want to order it, there is a seller called Sgt Knots on eBay and Amazon who sell milspec threads. I think they sell 4oz spools. One 4oz spool is probably enough for one cover. I use Sgt Knots and when I can find it military surplus UV resistant thread.
All portable machines are fairly equal (equally poor). Around here I would get one in a thrift store or Facebook market place, and not pay more than $30. If you do buy secondhand, especially at a thrift store, make sure it has the foot pedal and power cord. Those are often missing. Look to see if it has presser feet and a few bobbins too.
Best of luck!
Bulldogger