I've got a machine that's a drop-in bobbin (Brother), one that's got a front-load (Kenmore), and a Singer that I never use. The Brother is computerized, and the Kenmore is mechanical. I've found that the type of bobbin really doesn't matter. The auto-tension on the Brother is a lot easier than the mechanical of the Kenmore, though the Kenmore one lets me personally adjust tension for delicate fabrics.
A little secret is that MOST sewing machines are made by Janome these days. Brothers, Kenmores, Singers, they all comes out of the Janome factory with different stickers on them. The higher-cost ones tend to have fancier stitches and stronger motors, so don't really worry about brand so much.
Look for one that has straight stitch, back stitch, and zig zag. If you progress to buttons, you can upgrade to a machine that has one-step button stitches (I swear I went from hating buttonholes so much I refused to doing them, to getting excited to do them, solely because of the one-step on my Brother). The rest are mostly decorating stitches, and of those that aren't, you can live without them. I sew professionally, and I rarely ever use anything other than those three stitches. But for each additional stitch on the machine, you can expect the price to go up another $10-$25. Sure, it looks fancier to have all the numbers on the machine for all the different stitches, but save yourself the money.
If you want a really good machine for very little, look for vintage machines on Craigslist. Even the best machines now, unless you're looking at industrial, will be made using a lot of plastic. The heavy, built-like-a-tank machines of the 70's and before (especially ones from the early 60's and before) will LAST FOR-EV-ER. And they're much less than a new machine. My next machine will be a vintage one. With machines, you really can't get too old. Even the old treadles are amazingly efficient and durable. But they take a bit more skill than a mechanical one from the 50's.