The only letters I recall getting from MedEntry is a welcome letter, a letter about the workshop notification and UMAT trial feedback letter. I don't think they want to spam you because it costs them!
It is difficult to say which test is harder among the two (UMAT or GAMSAT). If you are an 'early developer', you are likely to do better in UMAT; whereas if you are a 'late developer' you are better off with GAMSAT. By 'early developer', I mean someone who has a 'tiger mom' or someone who goes...
It is up to you to decide if you wish to attend a workshop. You should go to this link under their website
http://www.medentry.edu.au/faq_workshops.asp#attend
Hope that helps
You can see a list of there workshop dates at UMAT Courses - UMAT Practice - UMAT Training
The 2010 packages began early August and you should enrol as soon as possible to get the most out of your package. I have already enrolled and chosen one of the January workshops.