Right now, D*’s own HR2x series is your only choice for an HD-DVR. All the various models are essentially the same, but the HR21 and HR22 lack the built-in OTA tuner of the HR20. The HR21 can accept the AM21 module for OTA capabilities, but the HR22 currently does not support the AM21 The HR22 does have a larger hard drive for about 80% more recording space
It’s doubtful that you can get a deal on a used one. D*’s new business models is that the receivers are owned by them and customers just “lease” them. The monthly lease fee is the same for all models (SD, HD, DVR or not) but you pay various up-front lease fees - $99 for an HD receiver, $199 for an HD-DVR. If you do find a personally owned HR2x for sale, it will likely sell for more than the $199 lease fee you’d pay to D*. Many “used” HR2x’s for sale on places like eBay and Craigslist are actually leased units. If you buy a leased unit, D will NOT activate it. They won’t come after you, but they basically treat it as stolen and they will attempt to charge the original subscriber for the non-returned equipment.