Welcome Marman1974,
If you could be a bit more specific on the type of diorama.....
Would you be modeling German, American, British, Japanese armor? The most popular armor scale is 1/35 (although there are many great 1/72 kits available)
It is, however, much easier to find 1/35 diorama buildings, signs and such. Do you have a certain "scene" you want to model? Verlinden makes outstanding resin buildings, scene displays and bases that can help you along.
As for the Armor, anything from Tamiya, DML Dragon, Academy and now Trumpeter are (in my opinion) great purchases. I have also heard some very good things about Revell-Germany 1/72 scale, if you go that route.