Fixed (NAS) or portable (expansion)?

NAS, the "wireless" requisite is so that any and all devices can seamlessly connect to it and do their thing (automated regular seamless backups, file sharing between devices, media streaming...). I am not looking for anything fancy (nor expensive) tho, I just want a simple piece of hardware I can connect to a power plug and just forget it there. No need for redundancy, RAID disks, nothing of that sort.

If you have an AP you can plug it to, I recommend the WD MyBook Live series. Cheap, good performance, and it runs debian :-P

i love synology! going on my second one! got the 1st for sale, if you are interested
Synology DS411j

+Fernando Miguel thanks! I knew about it already, but that's sort of a big cannon to shoot a fly, for the use case (and budget) I'm targeting :-)

i guess! 
i do love my new DS214play!
if budget is your point, check out their new budget series 214e/se

Got a WD My Cloud (the MyBook replacement series). I'm not very happy with the purchase, and they seem not to support some of the things My Cloud did (no ssh, no ftp), but they're under a discount today on Worten, so... I just hope I won't regret it.
Oh, having the seller telling me that those disks don't live much longer than two years doesn't make me happy either...

+Marcos Marado The ssh interface is undocumented. Do the MyCloud have the http://<ip>/UI/ URLs?

It does