I don't use PRTG but you could also have a look at Domotz for doing this job. It's $19/month for unlimited devices so very cost effective and supports SNMP and will alert on all device statuses also has quite a few other features you may find useful. I'm the CM at Domotz in full disclosure but thought it might help your use case...
Thanks for replying (4 months later) :)
I ended up with a StarTech RK812WALLO 8U Open Frame and some shelves from NavePoint and it's been working fine. That combo was around 120 and it's been working fine. I upgraded my battery to a rackmount and with the 8u, everything fits fine. I may at some point get a rackmount switch and nas to ditch the shelves.
Old photo as i've since cleaned up the cables: