For intricate/precise seams (think intersections and the like), a straight stretch stitch is great. On my machine (an older model Kenmore), that particular stitch will go forward a few steps, back two, then repeat the process.
For general seam joining, a zig zag stitch is also fine. Most machines will have your basic zig zag that is one stitch per leg. However, some machines will also have a three-step zig zag which is very strong and typically seen on commercially made lingerie. Each leg will have three stitches.
Sergers are also nice to have, however they are out of most people's price ranges and to be honest, they aren't really a necessity for stretchy stuff. I generally stick to a straight stretch stitch for bodysuits, and I just keep my serger around for finishing edges on fabrics before throwing them in the wash and for occasional hems on quick organza/chiffon projects.
Good luck with your repairs!