For all intensive purposes, are bh and mh the same? Even in combinations such as abh and amh? Amh seems more like a dipthong. Any help would be appreciated.
Thanks.

I'm just a beginner, but my tapes do seem to differentiate between them only sometimes -- bhfuil and mhaith start with the same sound. At other times, one is v- and the other kind of vw-. Of course, I might be imagining things.

Is it a difference between slender/broad?

Yes, bh and mh are the same.

ar bhó
ar mhodh

i mbó
i modh


...although they are given a little extra function. Consider in English:

Pidgeon

It's common knowlege in English that when a d and a g are written side by side like that, you've got a j sound.
In Gaeilge, their sometimes used to indicate dipthongs. Mar shampla:

babhdán
staighre

You'll just have to learn to recognise them, or know them already, eg. receipt plumber debris.

The phrase is "for all intents and purposes", Alby.