I have inherited an album project that was started on Sonar 8.5 Producer. I can either buy a copy of that, now out of date, or the latest Platinum version.
Question: will the projects from Sonar 8.5 be read/open/import correctly in the latest version?

Yes. Sonar, what ever version, is fully backward compatible. Old versions are not forward compatible though only because things in older versions are accepted by newer version. But newer version have features that older versions don't know about. I have wrk files and Sonar Platinum can open them.

Thank you John - I've been looking at the latest version for some time and it seems to fit my overall production workflow well so time to spend some pennies!

I think you will love it but it will require you to relearn it. Do read the manual. Almost everything has been changed. 
Sonar Platinum is so much more powerful and so full of features it will seem like a completely different program. If you get stuck this forum will help.  

From the look of the huge number of video tutorials available I pretty much get it already. Just getting used to where everything is is half the battle and the context sensitive mouse toolbox will be a welcome addition - more of a return having used similar in Cubase some 30 years ago.
I think for me it is getting familiar with and learning the sound and limitation of a standard set of tools that will take a little time - the workflow reflects much I have been doing for years but just so much more ergonomic.
Looking forward to it and will be sure to ask here if I get stuck on anything.
Thank you.

You are approaching this in the right way. I don't see you having a lot of difficulty. Really best of luck and welcome to the forum.  

The only problem you might run into is if the project references plugins that you don't have, such as retired products (e.g. Pantheon reverb) that are no longer bundled with SONAR. If that happens, first see about obtaining those plugins - some may actually be included with your version but not installed by default. If that's not possible (e.g. they are 32-bit DX or third-party plugins) then you'll have to substitute them with alternatives.

As Bitflipper said there were some different VST's and Effects in the older versions but they are all easily replaced with something else. They will show and be grayed out so you'll know approximately what they were.
 
You can download and try the trial version or simply make monthly payments if your short of cash. The other option is purchase a lower version like even Home Studio is a great version. I bought it even though I own the full version. I then purchased the Air expansion pack on sale for $64 which gave me more VST's power and Effects than I'll ever use. It goes on sale from time to time , stayed tuned in the Deal forum here. http://forum.cakewalk.com/Deals-f85.aspx
So it is possible to be in Sonar land for around $100.