Well, I assumed you going to try to do the extender thing fairly early. But don't doubt the importance of a proper LAN setup, It's important for online gaming as well. Plus, a lot of this stuff can done over WiFi. Plus, setting up the media share to use the extenders should only take about 15 minutes if you have LAN setup right. I think the wired part is really key for the incoming TV feed for the DVR machine. If you use the Ceton, having the cable feed directly into the machine will keep information loss to a minimum. The HDHR, this is where you use eithernet to the HTPC is important. After that, I think WiFi should be able to handle feeding your DVR recordings to extenders; it may not be able to handle a lot of feeds; I've only done at a time.
I would agree that the iMac is too outdated at this point.
The $500 is going to be very tight for what you're looking to do. Not impossible, but you're going to want to take advantage of any sale you can find. First, the TV Tuner cards you're looking at--Ceton and HDHomerun that accept cablecards--if you buy new, are going to run anywhere from $150 to $250 dollars. You're going to watch some kind of Media Center friendly remote. You may have gotten one with a Xbox 360; I did with the Pro sku shortly after the initial 360 release, and another with the HD DVD drive. You may only need to pick up infrared dongle. You may want to get a BT media remote so you don't have to point it at the HTPC. I'm assuming you're going to build this HTPC, so that means you need to get an OS disc and license. I'm not a big fan of the OEM versions, but you can go that route. They're more finicky on the machine you're going to use. But doable. That's at least $40. If you go retail, I would recommend going the Upgrade version route. Use the registery hack
so you can do a clean install; it's not hard, but you may want someone who's not afraid to mess in the register there to help. A retail Upgrade of Windows will run about $100. The nice thing, you get both the 64bit and 32bit versions. To save money, I wouldn't get a SSD drive. Just use a traditional HDD; I think a good TB should give you ample space to a lot of DVR recordings; I have idea how much you DVR. I have no issues with all my programs and DVR on a 500GB (now 750GB because the old drive went bad). But all but one show I DVR are watch once and delete. A quick search for Western Digit 1 to 2TB drives showed prices from $85 to $223. Needless to say, you can see how it will be tight.
Either tuner will be fine. HDHomerun is a network attached tuner. I doesn't attach to the PC. I think it makes it little easier to share tuners among several devices. I haven't used the Ceton, but I know it's possible as well, and don't believe it to be very difficult; at least it should be somewhat easy. The HDHR are great if you don't have an empty slot of space in your PC, like with a Mac Mini. Plus, I think the DNLA shows promise for expanding it's use. Both the Ceton and HDHR are stable. They their quirks, for most people, they're easy to maintain and reliable. You can't go wrong with either.
I think you have one thing off. No big issue about having the HTPC in the living room in the entertainment unit. Hooking it up directly to the Xbox is not an option. I haven't known the 360 to be able to passthough any audio or video from a device directly hooked up to it; the exception being the HD DVD drive, but not the same. I assume you meant a AV receiver.
The last thing, I would definitely try to have some expandability built into your HTPC. Rarely what our original plan to use it for stays just at that. I wouldn't be surprised if you digitize your media and distribute through your home or mobile devices using XBMC or Plex, and use more web video services.
Personally, I haven't build a PC. I'm good at replacing parts that go out. There are others who are really good with hardware recommendations. I would definitely check out Assassin's guides and hardware recommendation. He gives excellent and solid information.