I have a iPhone 5S, but don't want the watch. I wonder if I can still make online payments with it instead?
No chance. Payments need to be 100% secure. They need to be secure even if you jailbreak your phone, install malware on it, anything. That's achieved by having a special chip in the iPhone 6 that does all the work and that cannot be hacked from the outside. The iPhone 5s doesn't have that chip, so it cannot use Apple Pay.
Nope, the 5S doesn't have NFC so you need a 6 or the watch, which do have NFC
iWatch has NFC, but it doesn't have the chip that does the encryption. It needs an iPhone 6.