Nice bench.
You could use any timber you want really.
Ash would be at the lower cost end, (American) Oak about middle getting higher, and Walnut crazy expensive.
Wenge and Padouk would be pricey but nice. Wenge can be a very brittle and awkward timber to work but is beautiful finished.
Yeah (American black) Walnut seems to be getting very pricey, at least where I get it from.
As above, Oak is beautiful and can be made to take lots of looks, from liming (white) to fuming (dark).
The corner joints look mitred, but are probably not just straight mitres. They’ll be reinforced, perhaps with biscuits, or a loose tongue or something.
Would you buy the wood planed to size or plane it up yourself?