You are correct in that SolarWinds Orion runs on Windows and requires MS SQL Server. What I can tell you is that I have lots of experience with NMS systems but am by no means a Windows or SQL guy; however I have not had any trouble maintaining our relatively large install of Orion.
I don't anticipate an Orion appliance in the near future because the Orion software is very integrated and relies heavily on MS Windows technologies.
If the Windows thing is a complete deal breaker for SolarWinds (hopefully it isn't but I can understand if it is) then there are two products I think you should take a look at if you haven't already...
OpenNMS: OpenSource (full product available at no cost) Linux based NMS that does a great job of network monitoring, is easy to maintain, runs well, and provides some good flexibility. Support is also available through the OpenNMS Group and is pretty awesome.
EM7: Appliance based solution provided by ScienceLogic, I don't have much experience with it other than we evaluated it against SolarWinds Orion and it was a close competitor... we ultimately choose SolarWinds; however this seemed like a pretty solid appliance based solution.
Feel free to PM me offline if you would like more info on my experience with Orion, OpenNMS, or EM7.