An ETS150 will do the trick. Although, you'll have to choose between the /3 or /5. I have a Rotex 150 to do the heavy sanding and then I finish up with the ETS150/3. If you only want one sander, the /5 will be your best bet.
As for buying Festool, you can go a couple of ways. If you want to speed up joinery, go for the Domino because it's just awesome. If you want a better finishing experience then a sander is a good call. You'll need a dust extractor for it like you were talking about or it won't be any fun. They TS55 will always be a great first Festool because it is a joy to use, but if you're happy with what you're doing then save it for later.
I would also think about what you may want to purchase down the road. If you get one sander the the ETS150/5 is the one. Sorry, you could also do the smaller ETS125 models to. If you're looking to round out your sanding and consider buying two or three, then you'll want to make sure you make smart purchases now. When I bought the Rotex I questioned how much I would use it. I ended up using it quite a bit. Many may not want something that aggressive, but if you build things like end grain cutting boards then the Rotex is going to be your best friend. If you buy a Rotex then you'll want an ETS /3 model to compliment it. The Rotex 90 DX is another great sander that makes that setup the trifecta.
Regardless, if you buy a sander then plan to budget for a dust extractor. It's a worthy purchase. If you consider a TS55, then the MFT is something to think about to. I use mine for everything.
Sorry so long winded. Just thinking about the order I purchased my Festool equipment. I bought the TS55 first. I will repeat that the Domino is my favorite of maybe any tool I own.