1) It completely depends on the enchantment. I'm not very good with Sharpness 5, but it could maybe be around 7k perhaps? At least that's how I view things. Outside Silk Touch typically I add 1-2k onto the level of the enchant to determine what is supposedly a "fair" price and then adjust according to whether people go for it or not.

2) Yes you can leave the first line blank, after you create the sign if it is a correct shop input it will automatically put your name as owning the sign.
As for line three you can simply put "###" there, you're right that there's no need for a ":", both because you are not buying and selling, and also because you are not only buying from people. (Meaning that you are only selling to customers, not buying the enchanted item from said customers)

3) You'll want to hold the item itself and do "/iteminfo". This will give you the specific ID for the enchanted item for you to use.

If there's anything else I can help you with just ask.