If I upgrade to pwSafe 2, will I have to buy Dropbox Sync again?

No. As soon as you launch pwSafe 2 for the first time, it will ask your permission to bring over settings and safes from pwSafe 1. When it does that, it will recognize your previous Dropbox purchase and enable this feature on pwSafe 2 without requiring any further payment. You'll only need to re-link to your Dropbox account and your Dropbox safes will re-appear automatically.

Please don't use the "Restore Purchase" function in pwSafe 2 if you originally bought Dropbox Sync using pwSafe 1, as you'll end up being charged again. If you need to restore Dropbox Sync on a device which does not have pwSafe 1, you need to install the old app first, restore the purchase in it, and then perform the upgrade process to bring it over to pwSafe 2.

pwSafe 1 is not listed on the App Store anymore, so you'll need to go to the "purchased" section, which is accessible on your account, by tapping your picture.