The MCAT prep books can be helpful, but in some ways the DAT is a very different test. For example, on the MCAT the bio section is passage based, while on the DAT it is more straight forward. Also, the DAT does not have a physics section to the test, but does have a math section and a perceptual ability section (PAT). The MCAT books will help with your studying for the Bio, Chem and OChem sections, but you definitly need to pick up some DAT specific material. I would suggest starting with the Kaplan DAT book - it gives you a lot of the materials you get from the classroom course at 1/20 the cost. Kaplan is a little weak on biology so I would pick up seperate bio review book like Schaum's ouline of bio or Cliff's AP bio - or maybe your MCAT biology materials will be sufficient. To complete your studying I would then get one or more of the following: Topscore DAT (computer practice tests), DAT Achiever (computer practice tests), or DAT Destroyer (practice questions with detailed answers). I'm not sure about taking a GRE practice class to help with the QR section. The DAT QR section is very similar to the math section on the regular GRE, so if you are struggling with math, then I guess take the class. Otherwise, if the QR review material in the kaplan book isn't good enough for you, then pick up a seperate GRE math book. Anyway, I hope this info helps - I put some links to these items below. Good luck.
Kaplan DAT book
Schaum's outline of biology
Topscore DAT
DAT Achiever
DAT Destroyer