I have plastic rifles with scopes and spare hands. Depending on poses, you could either remove the hand and weapon of one of your riflemen (whichever one seems most appropriate) and replace with the bits I've got. Alternatively, you could simply remove the scope from a plastic rifle and glue it to the barrel of one of your rifles.
Because you're using metal miniatures, you don't get much choice about weapons, so your best bet it's just to paint and sort later. The plastics that we've all been using can be assembled with different combinations of weapons so you need to make decisions before you start.
Again, I have plastic binoculars I'm not using that you can add to models to be spotters or observers. The bottom line is, as long as they're distinguishable from the regulars, any models are suitable. I'd suggest using one of the poses you only have one of for the observer.
And yes, I am the King of Cheap.