xcode problems installing, apple keeps telling me to upgrade?

Ive just spent the best part of 3 hours updating my mac (because apple told me xcode 6 wont run unless I install 10.7.4). However, now Ive done that, apple now tells me xcode 6 wont run unless I install 10.10? Is that right? A whole new OS with no path to upgrade?

Im wondering can I use older versions of xcode? Where can I download these? Will I still be able to submit to the app store with older versions of xcode?