Do you plan to use the Launchpad for the machines that need to be backed up and Plex for machines that need to play media? If so that should work fine.
I have never actually used WHS 2011

My understanding is it is a variant of Server 2008R2. So you should be able to install Plex on it the same way as I have easily enough.
Now if you get the HDMI graphics card, you may find that GPU decoding does not work (like I found out today, read my posts in this thread a couple of hours back). Your media will still playback (locally), albeit using 40-50% of your CPU. Streaming it to iPad will prob max out the CPU and to other windows machines 10% ish.
To enable GPU decoding on mine I installed the Desktop Experience feature. I don't know if this exists in WHS2011, maybe someone else here who has it installed can check? All this feature does is enable Windows 7 style stuff on your machine, which happens to include letting your graphics card takeover video decoding. If you don't care about CPU usage, you can ignore this and the above paragraph
rp2000