2 ideas:
1) As originally stated: use "UTF-8, no signature" encoding when creating/opening your files. The "no signature" part guarantees that we will not attempt to save/open the file with a signature. As long as typing your umlaute characters never generates Unicode characters (i.e. only SBCS/DBCS characters are generated), then you should be fine since you end up with a file that has no Unicode characters in it.
2) Create/open your files with the "Western European (ISO-8859-1)" encoding ("Latin 9 (ISO-8859-15)" would probably work fine too).
With both of these methods, internally, these files would be opened in a window that uses the "Unicode Source Windows" font (yes, even when specifying ISO-8859-1). If this is a proportional font on your system, and you want to change it, then you can set the font from Tools>Options>Font, pick the "Unicode Source Windows" element, set your font.
--rodney