I game with Medievals in 54mm metal. Irregular is a superb resource. You don't mention what area of the Middle Ages you'd like to game, but two other sources could be Tradition (good for HYW, but expensive)
traditionoflondonshop.com
and Dorset have a marvelous range (again focused on HYW, but there are earlier types if you look around) that includes civilians and even animals!
dorsetsoldiers.com
Irregular also has some nice animals if you are wanting some objectives, plunder, etc.
For an earlier feel, and fabulous Robin Hood figures as well, you might take a look at Imrie kits:
imrierisleyminiatures.com They are some of my favourites, and if the era's right, I don't think you would be disappointed. They are expensive, but worth it.
Good luck and let us know how you get on--54mm is very eye-pleasing and so would be an excellent choice for a demo game. After handling them in metal 28's always feel pretty wimpy in comparison.