1. If you have an old P3D database, there is no need to rebuild it for an updated P3D version. The only time you need to rebuild the database is if you change your scenery. The log file will tell you for sure whether a build was successful.
Oh, and NEVER build a database across a network: that is begging for failure. Always build databases on your FS PC and copy them across to your client.
Having said that, going from an old version of Plan-G to v3.2, it is worth rebuilding the database, since Plan-G will now display your Addon airfields in yellow.
2. The black square houses the wind arrow. If you see a yellow arrow within then (a) you know you are connected, and (b) you know the wind direction.
3. P3D uses a different version of Simconnect compared to FSX, and requires a different setup. See http://www.tasoftware.co.uk/forum/index.php?topic=3795.0 for details. With all versions of Simconnect, your FS PC requires a static or reserved IP address.
BTW "index" refers to the index of the entry in your sim connect.cfg that you wish to use. For most users this will be the first/only entry, so the index value will be 0.
Bear in mind that since P3Dv4 is a 64bit sim, its Simconnect might not work on a 32-bit PC - you'll just have to suck it and see (they might have compiled it for both, but I have no way of knowing). As Joe said, WideClient will still work.