Please educate me on tubes

Since I know nothing about tubes, I have a few questions:

Vintage, re-issuem NOS, modern production (or if there are other types)? What do these types mean?

Is it safe to buy used? If so, what do you look for?

Should I buy as sets (pairs, quads, matched)?

Where do you get the most, least or doesn’t make a difference? Input, rectifier, output, etc.?

Thanks!