Sculptris is a free application that was purchased by Pixologic to act as a feeder program, hoping that people would get started with its simple interface, want more features, and then upgrade to Z-brush, which has them, along with a button that exports your Sculptris model into Z-brush. So I don't think you're going to get a whole lot more features or plug-ins added to that product.
I believe Z-brush has the sort of rulers you're looking for; here's a video that shows how to set them up: http://vimeo.com/58145135
Z-brush 4R6 (the current version) lists for $795 and works on Windows or Macs. (We can do a little better on the price). Using free software is a good place to start, but it might be worth spending a little if it will help you avoid expensive mistakes.