Casey, you can setup the APs either way. If you plan on using the LWAPP, those of course will need a WLAN Controller.
Without seeing a design on the facility and walls, its hard to say how many APs and what type of antennas will be needed. But with 20K sq. ft. in an open environment, you would probably be alright with 2 1242s with Omni antennas placed centrally in the facility.
It is much more than "I have an area this big, what should I buy?"
For any given area, you could have 2 users, or two hundred or two thousand.
For thos 2/200/2000 users, they could be causal surfers, or they could be using a custom, database-driven gut-wrenching nightmare of poorly-written crap.
Any division of those 2/200/2000 users could need various security levels, access methods, authentication methods, accounting methods requiring many VLANs.
That 100X200 foot space could be really reflective concrete (a nightmare of multipath) or it might be a psuedo-tropical rain forest of plants and small trees (a nightmare of absorbtion).
It could have really, really, high ceilings (a nightmare to mount & maintain APs & antennas, but good coverage) or it might have energy-conservitive low "fluffy" ceilings (think "micro-cells" ... lots and lots of APs).
The trend seems to be to go with central control, but there are still potholes-a-plenty, even with a "simple" (HAH!!) setup using this technology (search around this forum a little for one of the recent code "updates" for the LWAPP systems).
The bottom line is that you need experienced eyes onsite, and hopefully they will be bringing their best site survey stuff with them.
Any design advice you get from someone that hasn't tested your location (or, in the case of a forum like this even SEEN your location) should be tossed (except mine, of course ;-} ).
There are too many variables; there aren't even any good blanket rules-of-thumb, the site, followed by the budget, should drive the design and implementation. Chances are you'll end up with a LWAPP setup.
I am also in the midst of doing a wireless installation. 100 x 200 is relatively a small area I would agree somewhat with ScottMac. However I am currently working with one plant that is 300 x 100, you have 3 options roughly available to you, a LWAPP system which means you have to purchase a WLAN controller which cost roughtly 3 k not including software, you also can do two 1242ag atonomous systems that you would have to administer indepently or you can get two 2 1242ag's and use cisco netork assistant to help administer them, heres the link
I have found it to be a useful tool to help administer my AP's, over all to anwser your question, I would use 2 1242ag's with 2.2dbi dipole antenna's, also if you have POE switches available use them for power if not use cisco power injector. as for placing them best guess would to devid the building into 3rds, and place them in the center of the first and 3rd section. this should give you the results your looking for. good luck