While maybe overpowered for what you want, your best option is probably to run a small-ish vm that can do what you need, including running some real monitoring tools. I use either KVM, VirtualBox, VMWare to run them, though I hate the devils owning the latters. Usually just need a web server, database, the app, some libs, and off you go, but doing so in a VM makes it portable too. If you can't get a few gig of ram, some cores, and some disk space, probably barking up the wrong tree yet anyways. If not useful, nuke it, start over with another until you do.
I do this both for myself and customers, usually cloning or building like a linux box running ubuntu installed with various monitoring tools like opennms for your needs. Installing OpenNMS is really easy, as is NMIS, just install an ubuntu or centos in a vm on whatever box you have, 1-2 cores, 4gb ram, as much disk as you want to give it, they tend not to be too needy. Using with customers at times I need to up the spec, but I tend to run multiple tools as well, but still far less than what commercial tools require.
Any windows/linux boxes to be monitored install/enable snmp agents on, you can get most of that out of standard snmp system mibs. Enough at least to tell you if you need to look closer. Network devices tend to enumerate power/fans/temps, etc, linux it's a bit more quirky but can with the proper agents. Windoze snmp tends to be ok even with built-in snmp and adding a gpo to enable it. Macs probably do, but not a mac dude to care.
Open source there's OpenNMS, Grafana, open-ish tools like NMIS, Nagios, PRTG in limited capacities, but short list of apps for me to use. Anyone that can sell something does, and can't say I always blame them even if the cheap bastard in me hates it. It comes down to what it's worth to you, and a funny thing sometimes between super small business-ish up to billion dollar companies.
-mb