Nagios or PRTG or other monitoring tool Monitoring Setup

We need a monitoring solution which can do the following on Linux Computers:
Monitor RAM and harddisk
Check following services:
- Celecy inscances from our own software
- celerybeat
- gunicorn
cronjob monitoring
Monitoring of REDIS Queue
Size and Date of folders
- Warning when folder grows
- Control of daily imports
- Control of folder size
Monitor cpu
Monitor logins on webapplication

We are using PRTG for monitoring windows environments, but we dind't get it working for linux environments. Been testing Nagios but haven't got enough time to learn how to configure.
Also any other low cost or free monitoring solution which could fit our requirements is welcome.
If you could fit our requirements and can do the job, please contact me for futher information.
Kind Regards
Stefanie Gross