Yes you can use InstantCake on a HD to format it correctly for use with the HR10-250.

Without satellite cables and an active D* account, the HR10-250 will be useless.

You should be able to use the HR10-250 in place of the R15, but you will probably need to purchase a new D* access card for $20. If you want to switch back and forth between the R15 and the HR10, you would need to keep both receivers active on your D* account. I believe the current charge for each additional receiver is $4.99 per month. 

If you already have an HDTV, you might want to consider getting a HR20-700 or HR20-100 instead of the R15. These both have an MPEG4 encoder built in and will be able to receive all the additional HD channels D* is scheduled to start making available this September.

AFIK, the R15 will not be able to receive them. The HR10-250 definitely will not be able to.

Another thing to keep in mind is that either the HR10-250 or either of the HR20 models currently require two seperate cables to be able to use both of their tuners. D* is currently testing Single Wire Module (SWM) connections, which should allow the HR20 models to receive the signal for both tuners over a single cable in the near future.