Welcome! It really depends on what you're going to be sanding, and whether you're looking for shaping or finish sanding. My first Festool was the RTS 400 because I do narrow frames and didn't think a Rotex would work well there. Several years later I picked up the RO90 and confirmed my suspicions. But, the RO90 is extraordinary for the things it does best and I use it all the time. I've added the RO150, and, for big work - floors, walls, table tops, etc, you can't beat it. As I said, it depends on what you're sanding. I expect you'll do what I did which is to start with one, get hooked, and add others as your budget allows. I don't think you can go wrong with any of their fine sanders. Oh, and you'll want to get their dust extractor, too. With my RTS 400, I started just using the dust bag that comes with it, and that worked okay. Then I hooked it up to a ShopVac, but when I added the CT 22 it made a huge difference. Hope that helps!