Bree is a great starter area. The remnants of Cardolan and Rhudaur are very adventurous and it is near the Shire, which is of course a more well known area. Arthedain makes for a good base of operations for humans and there is even a chance for Dwarves to join the campaign. Southern Mirkwood is focused more on the Rohirrim and the growing menace of the Necromancer: but not a bad area to start in.
I personally like Gondor more and there is some good material on it.
Also you may have to make a choice between using first edition material and second edition MERP. First edition gives you the thinner booklets and loads of adventures, but the second edition material bundles some logical areas (Gondor, Arnor, Mirkwood) and you might fill in the gaps using the first edition MERP campaign modules and of course the adventures. The 1.5 edition (that's what I call it anyway) (Citadel of the Nazgul, Gorgoroth, some others) are among the best materials I have seen and worth getting.
The second edition books do leave out some of the material printed in the first edition, but I feel it has undergone quite a clean up and I feel I use those books more because they seem more connected.
The southern material is fine as well, but quite unrelated, but that leaves big gaps for a creative GM to fill in.
All of the material is great and gives a good base for adventuring in Middle Earth, but reading the books (never mind the movies) still provides me with excellent ideas and insights. Best use the MERP material as a reference but try to stick with the canon.