As far as cloud back up goes. Well . that's about as cheap as it gets . I see your issues though. You could also look at another choice called " The Box" . You can uncle Google that for the link . My employer is getting ready to go this route .
I don't recommend this officially but as I use Flex Raid ( this NOT
a back up per sa ) and really have nothing of monumental value on my server, I don't do any other type of back up. All my movie content was d/l on the net and all my tv recordings are re re runs. Yes, the other option would be a one to one back up and if your data is rather low in volume , say , under 10 tb you could go that way to . Up front cost is going to be greater and those drives could always fail to .
One of the huge advantages to a htpc is that you can build a very low powered machine that does everything you need for very little money. Advantage = FLEXIBILITY ! and simplicity . Low power , low noise , low heat and can be very small in physical size if you want . .
Another option , and a lot of folks do this , would be to combine your htpc and server into on big unit . If proper case and fans are chosen you can have it out in the open in a entertainment system and still enjoy a quiet listening situation . WHS 2011 will run almost (if not all) the same programs and software that Win 7 will run . It will even handle a tv tuner as well . I don't see the need for a third machine . Again , I would not be afraid to run my server as a dual purpose machine . It is rather large in size( freakin huge) as I wanted room for a lot of expansion so that is why I built a separate htpc.
There are hundreds of builds in this forum for you to do a model and I really love Assassins guides . It is how I got into this hobby to begin with . Proly would not have even attempted it with out them . Of coarse along with tons of help from this forum .