You probably need a professional choir... a church without a regular congregation, or with very few regular congregants, is unlikely to ever have a "parish choir".

Was there ever a choir?

Are there cantors for all the Masses? Are they volunteers or paid?

I run a professional ensemble like that now at a shrine, in fact — five pro singers (including me — I direct and sing). We rehearse once a week for 90 min. or so, then show up on Sunday a half-hour before the Mass, sing through most of the music, and then sing the Mass.
— rep: Renaissance motets, Weber proper-chant adaptations (in English), my own SATB Responsorial Psalm settings, and a repeated SATB congregational Mass setting (all the music is different week-to-week except the Mass Ordinary)

My old professional ensemble, back in Stamford (an octet, with me) didn't have a mid-week rehearsal; instead, we learned all the music in a 90-min. rehearsal immediately prior to the Mass that we sang. It made for an intense, but immensely rewarding, early Sunday morning.
— rep: Latin propers at Offertory & Communion, a complete Renaissance Mass setting (except the Creed), Renaissance motets, my own SATB Introit and Responsorial Psalm settings (all the music was different every week)

If your musicians are good enough, a Sunday-only ensemble could work well. These scenarios will fail without ace sight-readers on all voice parts.

As far as costs go, it depends on what the market will bear. Find out what the local Catholic cathedral and/or big Protestant churches with extensive music programs typically pay their singers, and use that as a benchmark for setting up a budget.