1) Is there any difference between PREPAR and Flightsim 2017??
Flightsim 2017 doesn't exists.
The latest version of FSX is FSX:Steam Edition. Then, there's FSW (Flight Sim World), which is a new product that is not compatible with FSX, which has been released recently by Dovetail Games, but is still in Early Access, meaning it's not finished yet.
2) Can my FSX planes & PANELS make the switch....??
You can easily import FSX planes and panels from FSX into P3D V4,
if all their gauges are in .XML. If a plane use gauges made in C++, which are 32 bit .DLLs, they must be recompiled to be compatible with P3D V4, which is 64 bit.
In real world terms, this means many of the freeware planes will usually work, but most of the commercial products won't, unless their developer has made an update, which might be free or not, depending on the developer.
3) Does PREPAR come with AI-Traffic ??
Yes, but it's similar to FSX default AI traffic. There are 3rd party AI traffic products that works with P3D.
Last which version should I get, I would think the one that sells for $2,300 is a bit too much, I have little or no intentions to fly military missions, just want to stay with the commercial side....
You surely don't need the Professional Plus version. The regular Pro version it's good for every user, and if you are a student/teacher, you qualify for the Academic version.