if its your first grow, try a strain like skunk 1, very stable and a great smoke yeild ok, could go for 4 plants in that space The 600w hps is ok in the 80 x 80 but could cover a 1m x 1m so you will get less yeild but at the same time the buds should be more dank you will need a higher airflow to keep the temps cool in a smaller room
A 600w will cover about 1.2m in lenght so you put the light in the middle and have 2 big osclating fans, one each end this will increas the airflow loads on help your plants Could do a scrog to max the yeild Or another ideal Make 2 rooms, 1m long and the other 60cm you can have some in veg and flower at all the times You will yeild some fresh bud every 2 mouths