I would tend to agree with Wookie, You should be able to do this without using the 500mw AP.
As for Range/Speed. This often depends on the terrain and foliage in the surrounding area. however in Wellington we have a client that is doing a shoot over from one side of the harboujr to the other and getining and average of 4Mb connects. he is using a 24dBi Grid and a Borg Twinfire.
Depending on how many clients you intend to hook on to the network you could go for a Mesh AP system with a 200mw card in it, alternativeley if you are only going to have a handfull of clients then an AP woukd be the way to go. We have some more 200mw AP's due in in a couple of weeks.
Cool. now that is something that never occured to me until wookie printed it. B or G / BORG
Anyway back from the giggle. B tends to offer more stability than G. If you have clients that connect using B then this will drag the network down to B speed. However it is handy to have G. G also has a lower power rating and is more noisy.
We may be getting some A,B,G AP's with the next shipment that are capable of up to 108Mb but they are not exactly cheap.
from what i understand they allow you to use A and G at the same time but will depend on the quality of the link.
If it was not allready obvious I run Borg