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Also, I’m way more bothered than I should be about the imperial versus metric thing. 1.5 inches is very close to 4 cm, but it’s not the same. In an effort to avoid analysis paralysis I think I’ll just go with the 1.5 inches, because it’s smaller, but I’m not sure that this is the right thing to do.
since you're living in the United States and have better access to imperial sized parts, i reckon it makes sense to go with 1.5 inches. back in the US, i have some 1.5 inch Grid Beam i bought from the original Grid Beam creators. but of course anyone outside the US should be using metric, as will i in New Zealand.
Fasteners
for 1.5 inch Grid Beam, the "standard" (according to the Grid Beam book, which i highly recommend until i port the content to gridbeam.xyz/guide) is 5/16 inch holes and 1/4 inch bolts. the best type of bolts (sold by the OG's) are "joint connector bolts" and corresponding "joint connector cap nuts", but anything works.
Saw
regarding sawing and drilling, i just published %bUTTrIJ... which goes into my deeper feelings, but to be more helpful here: i think you'll want to find ways to use your hand tools with jigs that enable you to mimic a table saw and a drill press.
keen to hear more about how you go, this is very exciting! happy to help any way i can. 