You don’t need to pay again for an app if you delete it. The iTunes app store knows that you’ve purchased the app. The little button still says “buy”, but when you click on it it will eventually say something to the effect that you’ve already paid for it and ask if you just want to re-download it.

You shouldn’t lose any data if you’ve synced everything you’ve entered on the iphone with the web app previously. Easy to check by looking online to see if all the data you expect is there. If it’s NOT here online, and the data is only on your iphone, when you delete the app you will lose your data. But normally you’re synced and when you reinstall your iphone app it will just sync with the online app and pull down all of your data.

Hope that’s not too confusing :)