I saw your post on the Request section. I am myself interested on that kind of idea of progression, even to the point that I worked on a set of knight attachments that go from a basic tier 1 up to a spikey tier 3.
My suggestion would be to use a many skins of the villager model, because a huge number of attachments here on Hive we design to fit that unit. Also with the different skins you could start with a regular villager, then at lvl 4 change to a more armored one and at lvl 8 use the most armored sking available.
Oh, btw. If you go to your profile at the bottom of the page, it shows reputation. So far two users (including me) gave it to you for helping or expanding the hive community. (uploading models, answering editor questions, rating maps etc.)
To give it, each user's post has a check and a minus sign next to it.