The oven I built is close in size to yours(24X30X62). For ventilation I installed 1/2" black pipe at the top. Seems to work ok. If your referring to circulation within the oven I installed a Dayton high temp fan to recirculate air from top to bottom.
As for how many elements you need it all depends on how fast you want the oven to heat and how well it is insulated. I used two elements totaling 5170 watts which heats the oven to 400 degrees in around 25 minutes. Also when determining what or how many elements to use look at the maximum amps they will require. You may need to run a new circuit depending on what your element requirements are. If you are looking to use the oven controls, most of these only turn on one of the two elements(bake or broil) so you might need a different controller(PID).
You can also try this calculator previously posted:
http://forum.caswellplating.com/show...ven+calculator
I've found this calculator to be somewhat off for the oven I built. It takes a bit longer to heat than the estimate given.