There is no easy way to do this with the exception of InCopy. It's going to
be nearly impossible to get them InCopy CS5, though.
We want to create high-design spreads in InDesign, the majority of which should not be editable by end-users (i.e. sales people), but in which they could add specific data points like client name, address, date etc.
A PDF form would probably do this, but you'd have to do the conversion in Acrobat with CS5
As you mention "variable length" fields, you will need InDesign or one of its sister programs to take care of the text reflow, deleting optional pages, adding table rows and so forth.
With a low volume and not too fast response time requirements you could of course also stick with manually handled InDesign, eventually supported by scripts.
One alternative is the mentioned InCopy, but that would mean additional software on the end-user systems.
Btw, you would not need to match the version of your InDesign CS5, later versions can also open templates created with CS5.
Another way would be an online solution utilizing InDesign Server. The end users submit the variables via a web form and receive a ready PDF.
Whether you implement that workflow yourself of course depends on your programming skills, the expected number of requests, your privacy requirements and the depth of your pocket - InDesign Server licenses have a noticable price tag.
Eventually you can also utilize SaaS Services, for example I've seen Harbs mention his printui.com.