I'm afraid that unless you sing your part EXACTLY the way the track's melody is pre-programmed (and the same way every time!), Roland's Auto Pitch feature is useless for how you describe what you want.

The Korg DOES, however, have a 'real' Auto-Tune function....

I use the real Auto-Tune extensively on studio projects (it's hard to find a record nowadays that DOESN'T use it!). It has two modes.. first is a graphical representation of pitch, and drawing tools to change it. Second is the so-called 'Auto' mode, where you either let it correct the voice to the nearest half-step, or have to tell it what key and mode the voice sings in, and it corrects to that scale.

BUT... you'd better not sing any chromatic passing notes in that mode - it will ignore them. And the chromatic retune mode requires you to sing pretty accurately in the first place before it can tune you up more... What happens when it gets it wrong? Horrible warbles, wrong melodies, and if overdone, the Cher 'Believe' effect (probably not what you want to hear in church!).

As a realtime effect, rather than a post trick for the studio, Auto-Tune is not a 'magic bullet'. It requires good technique before it is effective live, and even there you have problems with monitoring. It is VERY strange to hear yourself sing with your chest voice, but then to hear a retuned version in the monitors! Weird chorusing, and slight delay (it takes a few milliseconds to process the audio), it can be quite off-putting.

Take the money that you would waste on an outboard effect, and spend it on a good vocal coach. Probably money better spent that way...!