I would expect those breeds to be laying by about 24 weeks old. Are the comb and wattles nice and red and obvious?
I have 2 cheap chinese import coops bought from ebay, they are not ideal according to the experts but the chooks do fine in them. If you are concerned about nest boxes why not get a couple of sturdy cardboard boxes, tape them up, cut a hole in the front, line them with shavings or straw and place them under a tree/shrub/secluded place. You don't need one box per hen, likely you will find they all lay in the same place anyway. Another way to make the nest more private is to tack a black polythene curtain over the front of their existing nest box.
Coops tend to have nest boxes at or near floor level and perches higher than that.
A coop that is 3' x 4' will comfortably hold 10 large fowl sized birds.
Many people simply convert an old shed for coop purposes, add some boxes on the floor for nesting, perches at varying levels, perches should be made of wood approx 2" thick and rounded, and bobs your uncle, a comfortable house for chooks, that's easy for you to clean. Make sure it's leak free and draught proof.