ATI does have an HD version of their All-In-Wonder tuner cards, but I don't think it's under $100, nor have I ever seen any other HD tuner cards under $100.
Now, if you're not needing HDTV recording capabilities, and just want to record standard broadcast quality (digital or otherwise), then basically any TV tuner card on the market will get you done.
For a media PC, I would personally recommend ATI All-In-Wonder 9600 Pro or XT, due to it also having an onboard FM tuner. If an FM tuner isn't needed for your media PC, then any tv tuner will work (though I'd still recommend the ATI AIW line, especially one that comes with their Remote Wonder)
Windows XP MCE isn't a necessity for a media PC, I'm simply running XP Pro on mine. While not a necessity, XP MCE does bring a lot of nice features, however there's development projects going on that essentially turn any XP box into an XP MCE clone, giving you the larger text and menus that MCE has, suitable for TV screens. Media Portal
. I haven't checked that out on my media center yet, but it's something I plan to do someday.
The one glaring problem I have with XP MCE is the file format it saves recorded video to. It's not a portable format, meaning you can't, say record a TV show, burn it onto a disc, then give that disc to a friend so they can watch it on their own computer. I believe Jose found out how to change the way MCE saves files, however, so this may be a non-issue now, as long as you configure MCE beforehand, to save in mpeg/mpeg2/avi etc format. The ATI AIW I have can save in all those formats, as well as a proprietary (non-portable) format, similar to what MCE does.
Not being able to change channels without a seperate satellite remote, isn't limited to media pc's, as vcr's have the same problem.
I know that some satellite receivers have a serial port on the back, that you can use to hook up a PC to the receiver, and have the pc change the channels automatically through it. It requires some modification to the serial cable, as well as software that will support that. You could also use an IR blaster, which basically transmits IR data. As long as you teach it how to use the satellite reciever's remote data, you can basically tape it to the IR port on the reciever, and use the software to program it to change channels for you (either through the media PC's remote, or automatically, via a scheduler).
There's also PCI cards that essentially are the satellite reciever, though you'll need a subscription to the service to view the channels, and possibly even an adapter to be able to have the service's subscription card attached.
I haven't tried either of those. I really only have a need to record one show on a recurring basis, so it's easy enough for me to just have my reciever set to the proper channel beforehand.
Building your own media pc is easy though. All standard components work, hdd, dvd/cd (burners or not), sound card, network card, mobo, cpu, memory, etc. What makes a media pc different from a standar pc is in the tv tuner card used. ATI's AIW cards are a video card, and tv tuner (and FM tuner in some) built into one card. Some tv tuner cards are stand alone, requiring you to also have a video card in the system. Some cards have remotes, others don't. And some of the ones that have remotes, the remotes suck.
I've used two types of TV tuners. Pinnacle's stand alone tuner, that came with a remote, that only worked with the tv tuner software. It claimed that it could be programmed to do more (such as open programs), but without the abilty to move the mouse cursor and "click" the mouse buttons... just being able to open programs would be essentially useless if you can't then navigate around them.
The other type is the ATI AIW, and TV Wonder (ATI's stand along TV tuner card), with their Remote Wonder. The Remote Wonder has 6 configurable buttons that can be programmed per application, but it's main asset is the large center pad that you can use to move the mouse around on the screen, and right and left "mouse" buttons, as well as a "click and hold" button for being able to drag & drop items. It essentially negates any need for a keyboard/mouse, other than when you need to type something, though even that can be overcome with software that brings up an onscreen keyboard (if you're really that hard up to eliminate the keyboard/mouse entirely)