You need a machine that has the basic straight and zig-zag stitches, with a reasonably heavy-duty motor.

Skip the electronic ones and find a used manual model with minimal controls. Craigslist in your town, and ask sewing machine repair places if they have any.

Singer 337 is nice for simple upholstery and decor, as is the Singer 237.

If you can find an older Pfaff with the stitch select buttons on the top, I had one and it was really nice

Almost all machines have basic straight and zigzag stitches; but you won't need to splurge on lots of fancy ones.

(That said, I often suggest people get a few of the fancier stitches, because I have found that such things often motivated me to branch out in my sewing. If only to play with the cool tools.)

If you're getting an older machine, get one w/ a buttonhole setting. Tons of the newer machines have this as a matter of course (though maybe some of the simpler ones don't.) Again, this is because I think it's smart to buy some "extra capacity," just in case you want to expand.

Of course, an older machine without the buttonhole capability should be nice and cheap, which might leave you able or willing to spend more when you're ready to expand your horizons.

Snap on feet - that takes the frustration out of dealing with a machine for the first time.