Should I use a Column naming convention or rely on column descriptions?
Pick a standard, make sure it makes sense, and document it. There have been plenty of debates about this (e.g. whether the identity column in the contacts table should be called ID or ContactID, or whether the contacts table should be called dbo.Contacts or the unnecessarily verbose dbo.tblContacts), and you're never going to find agreement among your peers. Which standard you choose is not important; what is important is that you use it consistently, make it well-known, and get agreement from your colleagues that this is the way.
Your columns should be self-documenting (and not cryptic, abbreviated or obfuscated) where practical. I would NOT rely on the extended properties, but that's just me (google/bing for problems with extended properties and you will see plenty of complaints I would guess). External documentation / data dictionary is much more valuable to me - if your database goes south and so does your backup, it makes it that much easier to rebuild. It also makes it easier to describe your schema to junior developers etc. without giving them direct access to everything.