Wheel size is all personal preference. 26" handle fast but don't roll over obstacles as easily as say a 29er, that turns slower. The 27.5's are a balance between the two and a good compromise.
I would say look at the 27.5's as they are popular, good selections and you should find plenty in your price range. Ask your friends if they have a Mtb for you to try and make notes on what your impression was, likes and dislikes, etc. Then go and get yourself one and ride into the sunset, lol