Adobe Illustrator is the best option because graphics are vectors, meaning you can size and resize your images without making them pixelated. You can create poster sized images or cake toppers with the same graphic and the quality of the graphic won't be affected.
The biggest question is whether you should get an inkjet or a laser printer. It depends on your needs. If you plan on printing a lot, the toner in laser printers lasts a long time, but the replacement ink can be pricey. The color on laser printers is much more vibrant than an inkjet. However, inkjet ink is relatively inexpensive but plan on replacing it often. Inkjets also print photos better, so if you use photography in your printables, that is something to consider.