|Sorry, busy day yesterday and I did not have time to respond.|
Yes, you are correct about using some of the external antennas with home wireless equipment. You have to be very careful due to the fact that the output from most home models is so low you can lose most of your signal in the cable before it even reaches the antenna. But I have had some good results with some of the directional antennas for specific applications.
Anyway, for your senario, I would try a single Access Point near the center of the room if possible. That should give you plenty of coverage. I just installed a Belkin Pre-N router yesterday and was impressed with the coverage when compared to a linksys G router that was being replaced. Even when used with regular B and G laptops the reception was excellent. Far better than with the linksys installed. I set up the Pre-N router in Access Point mode and works very well.
I do not know of any software for setting up fee based hotspot, but I have to say that I have never looked for it. Must be something out there.
Check out this article on the Hawking website http://www.wififreenet.com/free.html
If your shop is fairly open, you should have no problem with single unit with it's own builtin antennas. It is certainly cheap enough to give it a try. Just mount the Access Point as high up as possible. From my findings this weekend, I would not hesitate to spend the extra $50 for Pre-N.
If you find you need to add an additional AP, it is fairly simple. Check out http://linksys.custhelp.com/cgi-bin/linksys.cfg/php/enduser/std_adp.php?p_faqid=622&p_created=1084218456&p_sid=K_8HrGvh&p_lva=&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9NTE4JnBfcHJvZHM9MCZwX2NhdHM9JnBfcHY9JnBfY3Y9JnBfc2VhcmNoX3R5cGU9YW5zd2Vycy5zZWFyY2hfbmwmcF9zY2ZfbGFuZz0xJnBfcGFnZT0xJnBfc2VhcmNoX3RleHQ9c2V0dGluZyB1cCBtdWx0aXBsZSBhY2Nlc3MgcG9pbnRz&p_li=&p_topview=1