Questions about buying the iPhone 5S


So I want to buy the iPhone 5S from the online Apple store. I just have a few questions to ask:

1.) What storage size would be the best to buy? 16 GB? 32 GB? 64 GB? (Previously had a 16 gb, but due to larger app sizes it might not be enough.)

2.) I want to buy the unlocked T-mobile version. Will this model work perfectly fine around the world? I hope no issues.

3.) Does the apple online store accept payments via paypal to buy the iphone? or do you have to use a credit card?

4.) Are there any other additional charges when buying the iphone? Suppose I buy the 32 GB model for $749, is there any other charges to be paid?

5.) Is the apple online store the best and cheapest place to buy the unlocked brand new iphone 5S? Or are there better alternatives like Amazon or ebay with cheaper prices?

Thanks in advance for your answers.