If you are buying a Tivo to record OTA for the subchannels you can't normally get through Directv you probably don't have any reason to buy a new one - it will cost a lot between the price of the unit and either the price of the monthly/yearly fee or the all-in to pay those fees upfront for life.
I'd recommend looking on eBay for a used Premiere with lifetime. Probably even a two tuner model would be fine - you'd have to watch a lot of subchannels to want to record three things at once! You can pick one of those up for about $150 - pretty much the same price you'd pay for a used AM21 off eBay these days.
I don’t know about the other guy but I often record more than one OTA show at a time so I need more than just two tuners for my OTA DVR since I like to have a tuner free just to casually browse. Hence I love my four tuners on my TiVo Bolt.