Best way to test a webapp on different versions of iOS/Safari?

I have a very simple requirement that I am sure is shared by many developers out there. I want to be able to test and debug HTML5 webapps on different versions of iOS/Safari without having to buy a load of old Apple hardware. I have tried simulators on OSX/XCODE but these run so slowly that they are almost unusable. I have tried online simulators but these seem to lack features that enable you to do basic debugging so are useless to me.

It seems to me that the simplest thing that could work would be a browser that you could configure to use different versions of the Webkit engine. So you would need to be able to download a bunch of old versions of the Webkit engine and have some way of switching between these. You would then need some way of mapping OSX|iOS/Safari versions to Webkit versions so that you could quickly reproduce bugs and debug them. So the browser would need to have Dev Tools too.

Do you know if anyone has done anything like this already? Or can you think of some better solution to this problem?