OK, this might be easier if i tell you what i have first and all i want really is upgrade to freenas.
I have a custom case, with a mini ITX intel atom d2700mud board with 4gb ram(the max), a 4bay HDD icydock (currently 2x 1tb mirrored zfs and 2x 500gb mirrored zfs), 1 mini 2port SATA controller, 300w atx psu, running freenas.
With this i use (loosely put) file sharing (CIFS/SMB and NFS for Raspberry pi2), Dynamic DNS, transmission, plexmediaserver(within finch). I have used owncloud but ive stoped now as most of what i wanted to do i can do with plex. think thats it.
95% of the time the nas is idle even with transmission and plex running but not used, hence why i don't want it to use much power while idling, at present the current setup idles are 47w. It holds music and videos and some backups of photos from my laptop. i do sometimes use it remotely hence why i want it on 24/7.
Plex has never been used for video playing only testing, mainly music now and then, possibly more used for downloading media at remote locations or for DLNA, but thats not used much either.
Speed is not important as long as it can do the job ie, i can if I wanted to, stream a video using plex to transcode and with my current setup it nearly maxed out, but not quite this is ok as it didnt effect playback. And it's unlikely to have anything or any one else use it.
as its a custom case (pic in thread already) i can make a new motherboard mount plate if needed (and not some dodgy thing held together with sticky tape a proper plate with mounts). It is currently mounted vertically and could take a board with max dimensions of 20cm wide x possibly 20cm long. or it could be mounted flat but it could only be 16cm wide but a possible 38cm long.
money, this is always difficult. obviously i dont want to spend more than i need too or spend money on stuff i dont really need (hence trying to discuss it here) but i still dont want to limit myself and end up with crap just because ive said i dont want to spend more than.......! ive been happy (or some people may say lucky) with my current setup for the last 3 years but i have now added a few more things, plex and moved to zfs etc) from just filestore and feel freenas will be better and easier to use.
Does that answer everything? must have is as you say dependant on what can be afforded. file sharing foremost, then its the the question of have to access it etc.