Do not quote me on this, but if I remember correctly, if you download an app that is incompatible on your OS but had a version that was, then it would download the latest version compatible with your system. I believe that is the case, but I'm pretty sure though that you can't purchase an application if the latest version is incompatible.
Of course, I might be wrong in one or more of these things. If you'd like, you could tell us what software issues you are talking about regarding 10.10, and we might help you upgrading if possible.