The technical term for this corner cupboard

Why not Maple or Poplar? Poplar is very cheap but pretty soft; Maple is harder and pricier but still not terribly expensive. Both take paint well because they have tight grain that doesn't need to be filled to get an even, unblemished surface. You could even go super cheap and use something like Pine, as long as you fill the pores with a wood filler and sand it flush before painting. None of these stain as well as more expensive woods like Oak, but it seems staining is out of the question for you.

As Rl Swamp Yankee said, wood is wood (at least in that the vast majority of it will support your top if the base is built correctly). Any furniture grade lumber can support quite a bit of weight, so go with a grain that will fit your needs. To reassure you, Poplar is used frequently as a secondary and support wood in furniture projects, and Maple is often used to build 300+ lb work benches.

Here's an interesting overview describing wood's ability to carry loads:

http://www.ehow.com/info_12003543_load-bearing-properties-common-woods.html

And a link for calculating the load-bearing capacity of a column of wood (although this looks like it's only for typical construction-grade lumber rather than hardwoods):

https://courses.cit.cornell.edu/arch264/calculators/example7.1/index.html

Simple really assuming you have the proper tools.

• Cut your 4x4's to 18 inch length
• Build a frame
• Attach to the 4x4 posts
• Cut wood to fit the surface of the frame
• Add lateral support to the 4x4 posts

To join 2 benches at a corner, figure out your corner angle and modify the plans to match.

Wooden Bench Plans Website