If starting from scratch, a strong and durable entry solution that will last you for decades is simply buy an 8x4sheet of birch ply, rip it lengthways then screw the two sheets together (from underneath) and optionally glue them to make a slab top of at least 2x18mm thick.
That will be very heavy and very very strong. No matter how hard you beat on it, you won't snap 36mm (about 26 plies).
Fit a record vice on the front and you have a usable bench top.
You can throw this across a pair of store bought saw horses - the metal toughbuilt ones are great - and you have a work surface that you can then use to build the frame to go underneath the bench top. Just bolt down through the top with slightly countersunk coach bolts.
The top will be flat and strong, but being only (high quality) plywood will be much cheaper than a hardwood top, much tougher than a softwood one, and you won't feel too precious about it.
The only thing that this kind of top may not suit is the use of hammer in holfasts. Those need thicker tops to work properly and would tend to splinter any holes you drilled through the plywood top to accommodate them
You can learn and buld on this for years, figure out what you like and then build a lifetime bench later