This should be above the line.

I think you must be England-based since you spell "practise" (the verb) "practice" - a common mistake in England where the verb is "practise" but the noun "practice" but almost unknown in the USA where both are "practise".

If you REALLY mean the great traditional ballads (centrally, the Child Ballads) of the English language tradition then beware of the excessive rigidity that accompaniment can introduce. Holding attention for 50 verses or more requires expression that I find hard to achieve with my limited guitar playing, and doing it without accompaniment requires considerable vocal accomplishment (not necessarily bel canto). The banjo is IMHO more restrictive than the guitar for this sort of stuff, and if you are going for the great traditional ballads it may be that starting by playing classical guitar is a more enabling route than chordal guitar.

If, however, you are US based and more into their recent traditions there then I'd say just pluck away - so long as you can tune the banjo it will help you sing in tune and many beginners find that hard (hell, many longtime amateurs find that hard).

Hymns and spirituals are a different kettle of fish and hymns themselves are treated differently in the USA from in England. In both in the USA, and in spirituals in England, the roots lie in the Afro-Caribbean approaches to timing harmony and expression and I would think you'd have to join a choir that rehearsed frequently to get the right feeling.

Here in the UK ecclesiastical singing is generally much closer in approach to classical singing (although from the revival period the Watersons did a quantity of religious material and more recently there was a lot of religious harmony on the Devil's Interval first CD) but the average church congregation will not teach you anything about singing apart from how awful it can sound. I have no idea what I could recommend.