Is it better (safer... less confusing...) to make the Master-key Sign (and Certify) only - eg. by using "RSA (sign only)"... or must/should I make it both Sign (and Certify) and Encrypt - eg. RSA and RSA ? Is Sign&C-Only more secure or doesn't it matter?
You only need to create a master signing key. The purpose of the master signing key is nothing more than to sign other keypairs which you use as your subkeys. Because of this, there is nothing you will ever need to encrypt using your master key. Security-wise, it doesn't matter if you create a master encryption key because you'll never use it, but it's unnecessary and kind of sloppy.
I'll definitively add an Encryption sub-key - but should/must I add a Sign sub-key too? ...Or could I just use the (full!) Master-key too sign stuff? (It would be a bother perhaps, but lets say I very rarely signed my files/mails...)
You do need to add a signing subkey as well, not just one for encryption! The purpose of keeping a master key offline is to avoid using it excessively in situations where it might get stolen. Signing everything with your master signing key completely defeats the purpose of keeping it offline, even if you sign things only rarely. The only time you should ever use (or even access) your master key is when you are generating new subkeys and need to sign them. As such, you need to generate a keypair with both encryption and signing subkeys, and sign that using the master key.
Note that there are secure smartcards you can get which can store your master key in an encrypted form and sign subkeys without you ever needing to put the secret key on your computer, just by plugging in the device. I suggest you look into one of those to help with managing your keys.