John is right.   I've sung in choruses for decades--used to call myself a choral gypsy but I've found a choral home--now almost 20 years. And choral writing sure does make you stretch your range. And perform all sorts of acrobatics with your voice--especially as a bass, it seems.   (Which is made immeasurably easier with a smattering of music theory--not much theory, but some.   Enough to be able to tell what what key a composer is in at a given point--especially somebody like Bach, who modulates at the drop of a hat).

I would think that if you sing in a chorus you don't need vocal instruction. You can learn an awful lot just by singing.

You'll probably extend your lower range just with the passage of time--I think most voice ranges tend to drop with age--but would be curious to know if others agree.   I've found recently that low D's and sometimes C's (2 leger lines below the bass clef) are much easier than ever before.

But extending your upper range is most easily done through falsetto--at least I've found that to be so. Again, curious as to others' experiences.