They should allow you to download / install version 17 if you are a new user. Some background:
When I worked for RISA (a few years ago now), they were getting ready to get rid of CDs and manuals and such and go entirely electronic. So, as a part of that they could no longer tell people with old versions, "Sorry, we can't send you an installation routine for that because we no longer have the CD's and we don't post those old installs to our website".
They vowed to always make those older installations available on their website. You might have to contact them, but that they'd always be there. Now, this was when the company was under different (much more customer focused) ownership. Therefore, they may have changed this policy again. I would ask them before you purchase to be 100% certain.