I think this is such a fun project and it will look great! I did a little searching online and it seems like you can just take the measurements to a local granite manufacturer to create the top. They will likely come out and measure to be exact and do the install for you so it's just right. Since the piece will be in the bathroom another thing to consider is the finish on the buffet itself. Is it well sealed with a quality stain? The moisture in the bathroom can cause havock on old wood pieces so definitely make sure the finish is just so before installing!

We had an old desk as a vanity in our old house. While a solid surface or stone is the look you are going for (and beautiful BTW!), ours had a drop in sink and the exposed wood was sealed with a marine varnish. We never had any issues with it but, it was in a powder room so, the moisture level was not there. Please post "after" pics. The piece is gorgeous.