as you can see I have made several level selection assets, I did this before I was informed of a more efficient method of creating buttons like these(making one blank square and using fonts). Will proceeding to use several items cost aplenty time or not so much?
Sprite as part of your buttons. What you have done looks nice. Use spritesheets to reduce the overhead.
as in use sprite for the disabled versions?
indeed. I use Sprites because they look so much nicer than a stock button. Just ensure the sprites are small but still good quality and use spritesheets.