Any of the old Singer machines with straight stitch and zig-zag will do, provided you use the right needles. They are good for quilting-weight cottons up through heavy denim ... Sunbrella is in that range.

One trick is to pause just before you hit a seam, with the needle down, and put a shim under the presser foot to avoid hitting the bump and stalling.

If you are planning to sew extremely heavy upholstery material - the stuff with the thick backing, puffy chenilles, car upholstery stuff - you need an industrial machine.

I just bought a Singer FashionMate 237 ($15 at a swap meet) and it's having no problems with repairing denims and sewing shirts.

Check at businesses that sell new machines, and repair shops, and Craigslist.

Homeybird, I'm not sure if you've already purchased a machine, but I bought a used Bernina 1230 about 20 years ago and it was one of the best purchases of my life. It has never given me a problem of any sort and is a real workhorse. I have since purchased a newer Bernina but will keep the 1230 forever. I don't believe you can go wrong with one of the older, quality brands, and (at least with the Bernina 1230) you can still get various feet for different projects. Just be sure to try them out at a reputable sewing machine dealer where they can answer all of your questions.

I have a couple Viking machines and I love them. They come in a wide range of prices and features. You can get a basic machine for a reasonable price or upgrade to a computerized embroidery machine. I do a lot of different things with mine from home dec and quilting to garment sewing and there really isn't much they can't do. Often you can trade in an older machine for an upgrade as your sewing needs and skills change.