a - If you have active maintenance, you should already have a V9 license available... I recommend 8.5.1 SP3 before doing the upgrade to v9.1 (latest SP5)
b - I do not recommend installing an eval version on top of a licensed product. There are countless threads on this forum detailing the upgrade successess from 8.5 to 9.1 - I believe you will be very happy with the enhancements... If you experience any issues, please don't hesitate to contact Support.
Hope this helps,
addendum to b - re: "box is NOT connected to the Internet" - You can manually register the key online.. If you have difficulty, please give us a call.
We actually recommend that you upgrade to 8.5.1 first, and then you can go directly to 9.1 SP5.
You will need to upgrade the license. Check your customer portal first for licensing information, and if needed, contact Customer Service. You should also be able to download 9.1 SP5 from the customer portal. CS can help you with getting the license to the right box.