Make your bench out of good quality, relatively dry construction lumber, reclaimed lumber or a softer hardwood. If you can, make the top a full 2" thick. There is certainly nothing wrong with using construction lumber for the top but the 2" gives you assurance that your holdfasts will hold without blocks and has numerous other advantages. In our area, there are many suitable choices available for $3.50 per bf or less. I paid $3. There are 32 bf in the top, so that's $96. The total cost of all materials would still be less than $200 for the most important woodworking appliance you have. If you can't go this way, just use construction lumber and add bearers.
Finding suitable material was the hardest part of building this bench for me. Dry construction lumber just isn't available here in the midst of douglas fir forests. The best looking construction lumber I saw was #1 & btr 6x6s. They seem to be generally available in higher grades, likely to meet structural specifications. I thought very seriously about buying some and resawing them. I am lucky that I have a bandsaw that will do it, but you could use a circular saw, cutting on both sides and finishing with a handsaw. You'd have to wait awhile for it to dry before constructing the bench, over a summer here. Reclaimed lumber would be a great option and that was my first choice. Unfortunately, it is so popular here that it costs more than new lumber. I feel very fortunate that I found a small custom mill with a kiln and I decided to spend the extra money to use premium lumber, but it wasn't necessary. Had I not done this, I probably would have made the base out of construction lumber and purchased two softer hardwood planks for the top.
There is no reason not to build this bench entirely from 2x6s if that is what you find.
I used 120 bf . You could use less but, given the price of construction lumber, why not add mass and rigidity?
Bottom line: You just have to see what's available at a price you can afford, just as a 19th century craftsman would have done.