ADSL line is not going to help you when you with 500+ clients, as the upload bandwidth gets maxed out very quickly by such programs as Skype Video, Torrents, etc, so unless you can combine (loadbalance ) adsl lines and target for say 20Mbit upload to start with? - no point in having for example adsl line with speedtest 17Mbit down but 600Kbit upload,
Also can you issue public IP addresses to the clients with the ADSL line(s)?
I want to start small to first connect the customers and when the client base grow I'll get Fiber.
The problem is I don`t know what equipment I need.