The best thing you can do is make a backup before you do the erase. That's because the erase all settings process will wipe out all the data and settings on your iPad. If you have a backup first, you can simply restore from backup and the iPad will be as it was before.
Now, if you are erasing all - and don't want to use a backup - then that's different. Yes, you'll have to manual load all your apps you bought from the App Store (either from the iPad or by transferring them over from iTunes). But, don't worry: Once you've bought an app, it's yours and you won't have to spend money on them again.
Oh, and those purchased apps will still be in iTunes anyway (or show as Purchased if you check that tab in iTunes). So, Apple will always have a record of what you bought and you can always get them back at no further cost to you. Assuming, of course, you use the same Apple ID...
Hope this reassures.
Marilyn