Porting an application (Robocode: http://robocode.sourceforge.net/)  to SaaS using Force.com
♦ Application
 A user can create her/his own robots
 A user can play the game using her/his own robots or downloading other peoples’ robots
♦ Expectation of Robocode as a SaaS program
 Each user should be able to edit her/his robocode
 A group of users should be able to play the game using their robots, so the system should have a gaming session manager
 A user can start a gaming session and wait for other players to join the session, the session starts when the desired number of players join the session
 A user can also schedule a gaming session for a future time, if at the time the session has the desired number of players signed up then the session starts
 A gaming request should indicate the level of the robot (how competent the robotswho will join the game is), the desired number of users to join the game session, etc.
 Instead of starting a gaming session, a user can search for open gaming requests and join the session
 Each user should have her/his own datastore to store her/his own robots, scheduled game requests, information on passed game sessions (recorded playing history to help improve the robots), etc.

My question is using developer's edition is it possible for multiple player (multiple session) to login to my game site ? I mean do I need to buy license for multiple user ?

I am completely new to force.com. After exploring little l am thinking of going with Visualforce for UI and apex for logic. Robocode is written using  java applets. So I am thinking of uploading Robocode jar file and embedding those applets in Apex Pages.
Is that a correct approach ? Or is there some better way to do this application say Heroku ?

Please give me some suggestions.

Thanks