The stalls are the whole ground floor of the theatre - they get less expensive the further back you go, and rear stall are cheapest seats.
The circle is the first balcony. Front circle is probably best seats in the house.
The balcony is therefore the 2nd balcony really.
Edinburgh playhouse is big. Its much deeper than it is wide. The stalls go back a long way and the back few rows of the stalls are awful seats - you're a long way from the stage, and unless you're really tall you can only see between the heads of the row in front. Front stalls are OK.
The circle is not so deep and even the farthest back seats there are OK I think.
The front of the balcony is high but good. The back about 2/3 of the balcony really feels a long way from the action. I've watched ballet from the back of the balcony, but really felt a bit removed from it all, and the sound is pretty poor up ther also.
If you get a chance to see anything at the festival theatre while you're there you'll find it's a much nicer theatre than the Playhouse. However literally hundreds of venues are used at festival time, and in fact you don't have to even leave the streets for a days entertainment.