Most, if not all, new machines will be able to sew out designs that you download. If at all possible, you want to get one with a USB port, not a card (looks like a card you'd put into your camera, BUT it usually has a propietary format that requires a special $$ writer box). If possible, you should go and use machines at dealers. Most machines will let you add text to a design, and possibly make the design a bit bigger or smaller. If you want to combine designs, create your own designs etc, you will need embroidery software for your PC. I'd look at Embird, which is pretty cost effective. Google it. There are several sites that will allow you to download a sample version to play with. Also, get the machine with the largest hoop size you can afford. 4X4 is pretty limiting.
Good luck!