Your chief concerns are not that important: the major hash functions are a) very cheap to run, b) already implemented in every language you can think of, and c)

Your chief concerns are not that important: the major hash functions are a) very cheap to run, b) already implemented in every language you can think of, and c) have collision rates of one in many trillions. MD5 is not recommended for digital signatures since it's been broken. SHA1 is under suspicion. But in most cases other than digitial signing of untrusted data, collisions are not the end of the world. Just use SHA1 or SHA256 and move on to more important problems.