Preparing Resin Items For Painting
Clean all resin with soap and water to take off the mold release. Use an old soft tooth brush or soft brush clean the surface. Glue - any decent super glue or plastic glue should work well. Don't use the dollar store version. I look at it this way - why buy the cheap version when you bought such a nice model to start with. You could also use any type of epoxy for things like the treads that have glue points you won't see. Epoxy tends to give a little rather than have a stiff bond.
For buildings we recommend using epoxy and mounting them on a base of masonite or plywood. Leave the roofs unglued to the building and then the interiors can be used during your miniature wargaming.
If a building piece is warped due to manufacturing, boil some water in a pot or transfer boiling water to a tray for larger pieces. Place the entire piece in the boiling water for less than a minute. Remove it and place it on a counter with a weight (like a book) on top of it. Once it cools and re-hardens the warp will be gone.
Priming with Gesso
Here is a link to a great article with pictures and details on how to use Gesso as your primer. My interest in this was sparked by very cold weather here and wanting to get some miniatures started.
Castings with Some Discoloration
The metal was probably a little too hot when they were poured resulting in some of the alloy separating out near the surface (sometimes the colors are brown/gold or bluish/purplish.) A good cleaning & priming before painting should help hide it.