Some clubs have a 'minimum barrel length' rule because they say 18- 20" barrels are "too loud." You'll find that a bit longer barrel may well help you swing better, it won't necessarily shoot any harder or pattern any better, but it might help in tracking fleeting targets. Spare barrels for 870s are widely available and not horribly expensive, they are fall-down easy to swap as well.
If you plan to buy used, study the platform seriously and learn your way around it before you go shopping. It's true that 870s are very durable, but some of them have suffered serious abuses of a kitchen-table "gunsmithing" nature at the hands of previous owners. Learn to evaluate 870s first before buying used, or find some help.
FWIW, I've been shooting 870s for going on 40 years and have yet to buy a new one.
Happy shopping,
lpl