Welcome.
If you aren't sticking with the 550F seat a 500/550K tank will go right on. 500T tanks are popular
Your question is a little hard to answer because we don't know what café means to you. I would recommend looking at the Project Board and Bike of the Month threads. Tons of beautiful café threads there with details on what tanks were used and how they were modified.
The Project Board also has a thread called Inspiration Bikes with hundreds of pictures that can provide, well, inspiration for you.