What Sean said is correct. You can have your fingerprint setup right now to only use for App Store purchases and not have to have fingerprint unlock enabled.
But, if I'm understanding correctly, are you saying you don't even want to have a passcode setup to unlock the phone? The only way you will be able to setup fingerprints for anything would be having a passcode enabled.
For example, I just completely removed my passcode and turned off fingerprints. I then enabled only app store fingerprint and it asked me to setup a passcode again. Even with the phone unlock toggled off, sliding to unlock requires you still to use your passcode to get in.
Bottom line: You'll probably be able to use the fingerprint for only Apple Pay, but there isn't going to be a way around sliding to unlock without a passcode.