Maybe went overboard this week and solved 3 different ways; first by substituting known letters (systems of equations and logic) and then solving for the remainder using a batch macro; second by substituting in similar fashion and appending/replacing; and lastly - a full solve using all possible permutations for the 8 characters & 10 digits. Probably room for refinement.
Fun fact - if you don't constrain the Sum to 5 digits (letting m=0) there are actually 25 solutions in total.
Logic-augmented brute force. Started out the complete brute force method and realized after a few minutes of processing time that it was hopeless. The key for me is realizing that this is also a puzzle--the M must be a 1 because there is no combination of any 2 non-repeating 4 digit values which could yield a sum of 20k or larger. That allows the brute force approach to have to deal with one less character in the solution and also removes any value in the namespace containing a 1. The whole thing executes in just a bit over a minute. Interesting one.