If I were you I really wouldn't bother with getting set on Sage (back-end or otherwise) until at least trying Win7 MC. It's nice.
That said, at least in XPMC and VMC, MC isn't very good at treating externally-recorded videos as recorded TV. You have encode them as normal videos and then throw them into your media library and browse the videos like photos. You won't get the nice metadata fluff that comes with recorded TV, for example, such as the series info, cast, rating, etc.
I'm not familiar with WHS as far as MC is concerned; however, AFAIK, MC is pretty much the same on any flavor of Windows 7 that has MC. If you put tuners on Win7 boxes you'll have no problem recording TV from MC at least for playback from the machine it is recorded on. You can playback from an MC extender.
I'm sure you know this but to be sure:
Are your STBs HD? Be aware of tuner limitations with Windows as a DVR. Instead of one kind of tuner (as with analog cable and analog antenna) there are three common ones for digital: ATSC (digital antenna), ClearQAM (unencrypted digital cable), and CableCARD (encrypted digital cable, licenced technology from CableLabs). Each has its own caveats, and you'll need to evaluate your cable provider's level of support.