WIMAX? why not just use 802.11 it is unlicensed freq. so the fcc is not involved, it would work fine for a neighborhood wisp..however whoever you have supply the backbone will need to be made aware you are going to resell the service.Typically I believe this is referred to Carrier Class Internet Service....I have seen cable companies telco and wireless companies provide for projects just like yours. As far as how much pipe you will need that is the most difficult part to come up with. You cannot come up with a reasonable answer for this without knowing some demographics above and beyond potential number of customers. For instance every neighborhood will be different, times people are home using the internet will vary greatly from house to house. I don't believe there is a magic equation to figure this with. It would have to be done on trial and error to start until you have reached a suitable medium.
If you planned to service a 50 mile radius you might would want to look into WIMAX. Regardless of how you go , know this , the internet market is getting ready to be turned upside down in the next 2 years. People should be looking forward to very inexpensive broadband on a wide scale. When I say inexpensive I am talking less than $15.00/ mo. in the very near future. An investment like you are thinking about would be a very shaky one at this point. At least according to some products that were released and showed at CES this year. It will soon be very difficult to compete in the internet market due to unused spectrum that was auctioned off early this year......plans for it include wireless internet like we have never seen before....and the telcos are buying up the licensing left and right, Verizon and Sprint are the big 2 .