I work for a large local government organisation who are about to embark on using SharePoint to replace our ageing intranet with an all-singing all-dancing collaborative sit
Not knowing anything about the UmbracoCMS, i would say that using a sharepoint system will allow you to do the things you need to do now as well as extend that functionality into the future. As an additional comment to those made above, keep in mind that you will want some kind of publishing feature for this external site. WSS and MOSS can accomadate the feature. Migration of content is also interesting since you can develope functionality on your current farm and then selectivly push these features onto the external farm.
To sum up: Though the setup is still a pain in the butt, you will have a much better administrative experience if your topology is homogenous. After all, this is what you current problem is anyway... why create more chaos?