Darmstadt is not as expensive as other cities in Germany (rent is around 300€/m). http://www.unicum.de/studienzeit/service/lebenskostenrechner/index.php?bundesland=8
http://www.studis-online.de/StudInfo/Studienfinanzierung/mietkosten.php
Americans are supposed to get a visa, but I know many who never registered and nothing happened - I don't know if this is the case for other nationalities, though.
The generalization that Master degrees from German universities are not as well-regarded as from US universities is not true - TU Darmstadt is well-regarded for engineering and science https://www.tu-darmstadt.de/universitaet/profil_1/rankings_4/index.en.jsp.