Ata for Asterisk or standalone IP-PBX


I’m looking for an IP PBX with 4 PSTN lines for a small company (~12 users). Most imortant of course is reliability, security and no audio issues (echo etc.). Another requirement is that teleworkers need to access it from different locations. Budget for this is only 700$, so any Switchvox/Digium Solution is out of reach…

As we already have a QNAP NAS (TS-659ProII) i was thinking about installing asterisk and attaching an ATA for incoming calls over the PSTN lines.
There’s a pretty big price difference between the cisco spa8800(400$) and cheap small brand atas like occtel sp4220 (200$), so does any major difference in performance or reliability justify to buy the cisco?
And if i use the asterisk server, how could a simple fail over solution look like, to avoid the losing the phones if the server becomes unavailable?

Or would you rather recommend a stand alone IP-PBX Box (like Atcom IP04 IP-PBX), that fits our requirements?