What you want to do is quite easy really. I do almost exactly what you are trying. I have a GigE based network and it works just fine for streaming to multiple places. I have a central PC with two cable cards as well as a HD Homerun. I can record 5 or 6 cable streams at once to the main PC. That machine is running W8 with WMC . The W8 machine holds the recordings for a bit and then shifts them off to my homeserver. Once there, any machine on my network can watch recorded stuff and they can access the networked homerun for live TV if they want. I don't have any extenders and have never used one either so I can't quote on that. I just have a bunch of mini PCs around for my TVs.
I actually like WMC for the most part. There are some customizations that can be done for W7 clients that make it better. The areas where it lacks are incorporating other services. Myself I have Amazon prime, netlifx, hulu plus, play on, playlater and Vudo and Epix accounts. The play on and netflix have WMC apps but the rest do not which is a bit of a pain.
There aren't a lot of options for DVR software really. MythTV and XBMC having working solutions but they still have issues. All the programs will take up the same amount of bandwidth so as long as you have the equipment it will be fine. I can and have fully saturated a GigE connection when streaming to multiple devices (two full blu-ray streams with additional network traffic). With that in mind I keep my high res content on my faster drives and regular TV recordings on slower drives. I now have server NICs in my machines that are teamed for a 2 Gb uplink which I can not saturate with just media streaming under normal conditions.
It all depends on how much you want to spend and how much media you need to push out. If you have a lot of media I would focus on storage more than anything. You will end up with a lot of content. I have about 4 TB of recorded TV, 3 TB of Movies, and 600 gigs of music and because of that I have a dedicated recording machine, dedicated HTPCs, and lots of storage space.
I think I answered your questions and then some but I'll be happy to answer more or clarify. My media setup is crazy considering I live by myself in a two bedroom apartment with 4 TVs.