Well, I have worked on a lot of projects to find arbs.
I guess the other bookmaker is pinnacle.
the XML feed of pinnacle allows the refresh every 5s -for incremental updates-, I have a module that can reduce this refresh rate to less than a seconds, for this, we need a client/server solution.
we can use amazon cloud for the free instances.
We can try the inplay as it is most likely to offer arb opportunities.
you have to decide which sport, which markets etc...
It is extremely tedious to make a generic app for any sport.
we can start by soccer as the first milestone. tennis for the seconds milestone.
Other sports need more funds as I know the complexity it involves.
Or, if you choose the inplay-where more arbs are available-, we can make it for any sport.
The most complew part is how to make association between team/player names from both pinnacle and betfair as they are generally written differently.
Anyway, drop me a line to talk about the project. I can suggest many other things that might interest you.