Hi Gav,
I can't say I have had any issues with opening V14 in V15 but haven't tried a V12 one and rarely print plans anyway so would not have come across anything odd - yet.
A agree that 20 min seems a long time for a Basic render, unless it VERY complex inside. I could understand it for Advanced maybe, although my models are very complex at times and don't take that long. As part of the beta testing of V15, I installed it on my wife's Win7 (Home Premium) laptop and was very impressed with the effortless installation and speed with which it worked. The laptop is nothing special in itself, and I don't think it even has separate video card memory but I had no problems moving around rapidly and rendering quickly, so if you are going for a better version it should be even better.
I think the V15 download issue is the one that comes to existing V14 users by email, to obtain a discount on V15. It is still showing the V14 file from 12 months ago. The link you have posted goes to the correct one. Should be fixed soon I hope!
Allan