For the machine itself almost any router will do it, bigger the motor the better.
I would say try and get a bit with replaceable inserts. The reason being that you might get inclusions of grit or metal in the slabs, once you get a chip in a big expensive router bit you can`t fix it and you are going to be pretty annoyed. Also flattening a slab is hard work on the cutting edges so being able to rotate the inserts is a very good thing.
Like this thing.
https://www.amanatool.com/rc-2250-i...-2-dia-x-1-2-x-1-2-inch-shank-router-bit.html
I know CMT do some replaceable tip stuff but be careful because some cant be mounted in hand routers and have strange bolt in shanks etc.
Ollie