I use OpenOffice Writer to check spelling, reduce the MS stuff, and on some magic way I have yet to understand, it converts all the punctuation to symbols that the Mobi Converter understand.
The software makes it very easy to switch from en_us to en_gb and it can use multiple dictionaries. I use a SF dictionary to hold strange SF terms, I use a Bernard Cornwell dictionary to hold all the Saxon/Gaelic/Arthurian names and places, I use a Pulp dictionary to hold all the weird profanity and grunts, I use a Language Dictionary to fold Foreign words and phrases, and I have a Slang dictionary to handle the multiple contractions authors use.
If I an editiamn a SF Book, th eactive dictionaries are SF, Slang, Language, and the default OpenOffice one.
The one fly-in-the ointment is the software won't find or replace a ^p/CR-LF that Microsoft uses.I still need to pass the final draft back to Word to change all the CR-LFs to ^l/CRs.
This last pass doesn't load up the document with the MS formatting garbage that swells a straight Word document.
Here is the link to get the free OpenOffice doenload:
http://www.openoffice.org/