I am perhaps exactly in your situation, never learnt German at school, and though I've visited on holiday and on business, and can just about manage to order a beer and a meal, not really progressed further than this.
I bought the second book you mention a long while ago, and found it interesting. Now that I know more about the genus, it is very interesting, and the tables of comparison and the photos help the explanations along.
I also bought Vol 1 of Die Gattung Mammillaria in February this year, and I have found it fascinating. I find that after a while, I can pick up most of the important words, and the location data and climate is pretty understandable. If I get stuck, I revert to Babelfish or Google Language translators, or a good online dictionary.
But do I assume from your post that you now of a source of the 2 volume "Gattung, Mammillaria", 'cos I have been looking for either Vol 2 on its own or both volumes for a godo while now.
Not everyone accepts Reppenhagen's classification, and his "rediscovery" of plants to which he has allocated old and possibly undocumented names. But you cannot doubt the breadth of his travels, the range of plants that he saw and documented, and for that alone these books are invaluable.