I did mention this in my original post, but I will say it once more - I don't have a planer/thicknesser, table saw, circular saw, mortising machine. I have a drill press and a couple of drills (cordless and corded), plus a metalworking lathe (which I didn't plan to use for woodworking - one day I'd love to get or build a small woodworking lathe - perhaps a treadle lathe on the terrace?).

Spending a 100 EUR for a thick laminated spruce board that I would have to glue edge-to-edge to get the width, vs 200 EUR for a store-cut laminated beech panel which I would have to glue face-to-face to get the thickness, vs a single board which I would probably have to drive far away to obtain. 100 EUR or 200 EUR is not important, I'm more concerned with how tricky the build is going to be and how happy I will be with the result. I can always ask a seller to help me load, and a neighbor to help unload / handle it in during the build. Hardwood (beech) is traditional, and recommended by many (although it is supposed to move a lot), but people have been using pine benches with success (e.g. I just read Vic Tesolin's "Minimalist Woodworker", and he advocates a pine bench mentioning e.g. ease of flattening it - even though he could afford any kind of wood for the top, as on the photos in the book all his tools are fancy top-quality stuff). With both of these choices, one concern I have - are those commercially available laminated boards/beams have the individual pieces arranged with proper grain orientation to minimize warping, and also with grain running in the same direction (for avoiding tear-out when planing them flat)?

@Mike Stenson
I have heard advice not using oak (unless you already have it, or can get it easily, for example) due to it being open-pored wood (trapping and embedding metal chips that can randomly land there from my metalworking activities) and prone to splintering - therefore I'm not too eager (yet) to drive 700 km round trip to Hugo Kämpf's sawmill (although I did drive 800 km to get my lathe, so if it's worth it, I'd do it). How did you find it so easily, though? Maybe I can use your search skills to find something closer to me (I'm in Luxembourg - Steinsel to be exact), who would sell a wide and thick beech board. But Derek had raised concerns with drying that thick board?

@Oskar
I just saw your post after I wrote all of the above, and your advice already answers some of the questions/concerns I have. I'm using hand planes only and according to your advice will have to rule out BSH. I'm also not sure about the beech panel which is laminated from many pieces, what if those panels also have grain running in all directions? If I remember correctly, Schwarz mentioned Douglas Fir as quite suitable material for a workbench.
The construction lumber you bought, what is it called in Germany - Vollholz? Is it a thick solid board, not laminated? How wide was it? How did you find that local saw mill? I think I should follow your way (and Mike's advice) and find a saw mill not too far from me.