There are no known plans for Comcast to start selling Motorola DVR's. The only Comcast supported device that you can purchase is Tivo. A Tivo with a Comcast supplied cablecard can get On Demand in some markets, with more being added all the time.
Let me add: There are cablecard devices that you can connect to your computer.
top of the line tivo that records 300 hours of hd with four tuners with lifetime sub is 500. In the long run you save money if your renting a dvr in about 3 years you will recoop your money. If you want a lower end one they are available.