They should have a legacy or discontinued section that shows info on older models. I can tell you that the TD-10 was their top of the line about a dozen years ago, although subsequent less-expensive models often had more advanced features and better sounds. The TD-20 replaced the 10, and now the TD-30 remain the top model... and is priced accordingly! Apart from that, I really don't know where the other models - past or present - fall within the line-up.
Try the forum at www.vrdums.com
, they'll know for sure, possibly have some used kits for sale?