I love Ryobi's 18v power tools and have a huge stable full of them (although I now recommend Ridgid for people just starting out with 18v tools). However, the Ryobi 18v jigsaw sucks! The chuck on them won't hold the blades straight, which means crooked cuts, especially when using a fence. Get a 120v Milwaukee, Ridgid, Makita, or Dewalt instead (if that's the only tool you buy).
Seriously, you will be wasting your time (and money since you will destroy wood and wheels on wood) using a Dremel for cutting wood. If you want to work with wood, you need woodworking tools. Would you try to cut a cake with scissors?
If you can afford only one saw, get a jigsaw. It is the most versatile tool for cutting wood. You will also need a drill. Again, if all you are going to get are the saw and the drill, especially if you won't use them much after this project, get 120v so you won't have to spend money for batteries and a charger. 18v tools are great but not cost effective unless you have a lot of them (like I do).
I minored in Industrial Arts Education in college and have worked in several cabinet shops (and a machine shop) so I know what I am talking about. You need the correct tools to the job correctly.