Internet Cafe - what do I need?

Hello,

My situation is as follows: a good friend runs a small cafe and has decided to provide paid-for wireless Internet access to customers. At present their ADSL router is doing dhcp and firewall. The owner has a laptop and there is another PC on site. Both connect directly into the router via cat5. There is no wireless AP on site at present.

The ideal situation would be that people can join the wireless network with any old device, when they try to access the Internet they are directed to a web page which says ‘to access the Internet you must first go buy a ticket from the desk.’
When a ticket has been purchased the customer can enter a code and browse the internet until their data limit is reached.

Is this the sort of thing that RouterOs can do?
Would it be better to build a box and install RouterOS or just buy a RouterBOARD product?
Which RouterBOARD product would be most suitable?

Thanks in advance!

HOTSPOT is best for your scenario.You can configure UserManager [Builtin small radius server in Mikrotik] to create prepaid voucher/tickets. When the user will try to browse, he will be redirected to your page which you can customize according to your requirement. After entering Id/Password, he can use his internet, and after expiry/usage, he will be disconnected.
Some links that may help you in achieving your goal:

http://aacable.wordpress.com/2011/09/12/mikrotik-hotspot-quick-setup-guide-cli-version/

http://aacable.wordpress.com/2011/08/09/mikrotik-pppoe-server-with-user-manager-pre-paid-billing-system/

http://forum.mikrotik.com/t/sample-hotspot-page-sticky-please/23665/1

RouterOS or Routerboard, both can fulfill your requirements.
If you are going for RB, If you are talking about 20-30 users , I guess RB 750G would be more then enough for you.