Great answers but there was one point I didn't see mentioned so I thought I would chime in.
The work surface
That's either pine or fir. A very soft wood, so even without cracks it will dent and scratch easily. Nothing less than epoxy will chemically seal it for more than a few uses. The stuff used on bar finishes is extremely hard, but a dough scraper will be able to scrape it if you are not careful. What will most certainly happen is the finish will fade unevenly where it is used most, so go with a matte finish. It's also quite expensive and not food grade. If you do use it, you will need extra for the cracks.
Offcuts of other stones or creating a laminated hardwood surface would be a better option. Here are a couple options
Laminated hardwood: I've made working table tops out of 1"x2" maple which came out nicely and took a lot of abuse, but it will take a while to finish if you don't have a surface plane.
Soapstone: would be the best choice for a stone surface. It can be cut with a diamond blade; extremely dense; chemically inert; you can sand it as smooth as you want; extremely hard; heat, even large amounts of heat won't affect it (think hot baking stone). It can be scratched but scratched can be sanded out easily.
Marble: is soft and may be easier on your hands. Chemicals and residual water will stain it. It can be scratched fairly easily, I've never tried sanding it but I don't think it works. It also doesn't handle heat well.
Almost any other stone would probably need to be cut for you.
Stainless: Easy to clean, good stainless steel (ex 304) is resistant to many chemicals and won't rust at room temperatures with typical household chemicals. Just make sure it is a thick enough gauge to handle the compressions of the kneeding. I wouldn't consider something > 14ga
I didn't see many photo's of the base but you may want to add diagonal cross members if you do not have them. On a residential kitchen counter, the plywood box helps counter lateral forces common in kneading dough. An open table with all vertical and horizontal members doesn't provide the same support.