As for your first question, it's perfectly possible to use your Raspberry Pi as a media center, I've done it once and it works just fine.
How I did it? Well, I've flashed my Pi with the XBMC distribution. You can use either the ones based on OpenELEC or OSMC.
The flashing process can be easily achieved through NOOBS, which you can download here.
As taken from the docs, to flash the drive do the following:
To set up a blank SD card with NOOBS:
- Format an SD card that is 4GB or larger as FAT. See instructions on
how to do this below.
- Download and extract the files from the NOOBS zip file.
- Copy the extracted files onto the SD card that you just
formatted so that this file is at the root directory of the SD card.
Please note that in some cases it may extract the files into a folder;
if this is the case then please copy across the files from inside the
folder rather than the folder itself.
- On first boot the "RECOVERY" FAT partition will be automatically resized to a minimum, and a list of OS's that are available to install will be displayed.
For more information on that process, you can check this link.
After that, you'll be able to install and use the XBMC distribution. First, boot your Pi with NOOBs, then on the selection screen, chose either OpenELEC or OSMC and then press
Install. After that the wizard will prompt you to confirm, press
Yes. Now you'll just have to wait until you see a message like this:
OS(es) Installed Successfully
Now click on
OK and your Pi will reboot on XBMC. Pretty straight forward.
This tutorial was taken from this link. You can check it for more information on how to use XBMC, and how to mount network drives such as a NAS.
Ok, now for the real question... If you can run it as a media server AND NAS... Well, I believe you can achieve that through the UPnP server option on XBMC.
I've also found this tutorial, which uses MySQL to sync your medias on XBMC. It's probably best for you to use your desktop or another dedicated server to host the medias though. You can also achieve that following that link.