I have a Bernina 800DL I bought new in 2004. Shortly after I bought it I had to take it in for service, as the belt slipped. I haven't had it serviced since. It has 2 oil points & that's it. Not too hard to thread. It's color coded for easy threading. I use it a lot & have sewn several layers of fabric on it at a time. It sits on the end of my sewing cabinet, ready to use. It's great for sewing stretchy fabrics, like knits. I also serger jeans fabric with no hesitation. This machine is a work horse.
As far as a good investment it all depends on how much & what type of sewing you do. You may be able to get a good used one at a Bernina dealer.
Look over the stitches on your machine. There may be one that is overlock. If not then do a straight stitch & go back & do a zig zag, like Lynn said.