Thank you for your comments

Thank you for your comments Mike.

To your question: We are a family of 6, and I believe that we usually will run about 3 extenders at the same time (if we also use an extender for the main living room), and the content will likely be mixed SD and HD at the moment, but I would like the setup to be able to handle several extenders running HD at the same time without lag.

Could you recommend a suitable i5 cpu, motherboard and RAM for this setup which is balanced on our needs ? 

As I mentioned, the HTPC server will most likely be placed in the loft of our house, so the size of the motherboard can be whatever size you recommend – maybe a full ATX motherboard would be the right choice ?

I´m grateful for any suggestions.