If you want to use OTA then SiliconDust HD HomeRun is one of the options. If you are going to sign up with Comcast, then Ceton InifniTV4 or Silicon Dust HD HomeRun Prime are your choices. you can mix and match however you want to get the most coverage.
There are no PC tuners that work with sattelite TV, unless you want to rent boxes from sattelite, and connect them to HTPC using archaic tuner set up with IR blasters so that HTPC can change channels on the boxes.
As far as the actual build, there are tons of options, it all depends how much you want to spend. I just built a "DVR" for my sister and it cost me under $400 for everything. I prefer AMD, since they get the job done with minimal costs. Some will steer you towards Intel. However, you pay for that.
AMD Athlon II X2: $30
ECS AM3 mobo - FAR
8 Gb RAM: $20
750 Gb HDD: $40 (roughly good for 60 hours of HD recordings)
Case with PSU: $40
Rosewil Remote: $10
Ceton InifniTV4 quad tuner: $150
Win 7: $30
If you have any of the parts laying around, like older HDD's, DVD burners/drives, cases, PSU's. ect, you can re-use those.
I know they will eventually want to connect other TV's in the house. I will have to upgrade them to Athlon II X4 or Phenom X4 for $50, and Media Center Extenders ($100). They can add external hard drives to expand the storage capability.
6 TV's in the house on FiOS and we only pay $4.99/month to connect them all!!! Power to the CableCard and WMC7!!!