Hmmm, expect extremely popular regular circles to sell out 30 - 40minutes when the event starts. Larger or famous circles might rack plenty of stock but still I bet about an hour for them too. Ask Forza about his knowledge of popular music circles :V. ZUN last year sold out in about an hour if I am not mistaking.
As Reitaisai rules go, you're not suppose to be in cosplay costume before the event, at least that is what they ask you (and quite a lot of people ignore). Cosplaying is set to a fixed time somewhere close to noon where the participants have plenty of time to change / dress up. I wouldn't be worrying about this part much. It is probably most leisure moment of the event as far as I have seen. Last year I had it easy. I just put on an afro, 30 seconds work. :V
Planning a route to each booth is highly advised for two reasons: Enjoyment and efficiency. The way it is: there are halls 東-1 2 3 on one side and 4 5 6 on the other. The two sides divide the circle listing into Latin characters A 02a etc or Japanese あ 02a etc. All hiragana marked circles are stowed into one side of the building and the normal character on the other side. So if you have 5 favourite circles at hiragana and 5 at other, you might want to first handle one group before moving. It won't be a disaster when you miss a booth or go random through them, though you don't want to cross sides all the time. Once the map is up, you can see what I mean.
Unless of course there are circles you must visit because of sell-out danger. Then well yea, unless you can get them in the stores post-reitaisai, I wouldn't really stress if I were you. You could always ask Forza to deliver them later on.
My own personal goal: Set a few booths to visit for greetings. Stroll a lot and enjoy. I usually don't buy much from Reitaisai so for me the enjoyment of walking around and observing is quite delightful.