What parts are recommended for an upgrade?

On the apple website, I think there is a way to install older versions of macOS. For your specific iMac, I’d put macOS High Sierra or Sierra, which I think should work with your iMac.

been finding it hard to get reliable info on the mid-2009 imac (aka the education edition). some say the cpu is socketed, some don't know, but mine with the mainboard mac-f221ea9 has the T7750 cpu soldered in place and not with the zif socket for sure. the nvidia 9400 is also soldered and unchangeable. so open it up to give it a clean, try and find some sodimm ddr3 1066mhz ram with 7-7-7-20 timings (or lower if you can, i have both micron and hynix sticks that have those timings), grab a useable ssd (there apparently used to be problems with certain controllers, but the posts i found talking about that were from 2015 so most modern ones should be ok if they're backwards compatible with sata2 [you'll still get benefits from sata3 capable ssds purely from the random read/write IOPs, lifespan etc.]) and keep in mind if you replace the optical disc drive, i think that port is limited to 1.5gbs instead of the 3gbs; main sata port should run at 3gbs though, but the nvidia m79 chipset can be a bit fecky with certain (older?) ssd controllers, again, info from ~2015 so might not affect much that came out after the samsung 860evo (was on macrumors.com somewhere)

!BUT! here's the good news! you can run catalina with a relatively easy hacked patch! (montrery doesn't work yet afaik)

http://dosdude1.com/catalina/

and if you can migrate away from the mac ecosystem since you only want imovie, if you want to install an official, still-supported and security patched copy of 64bit windows ltsc iot 21h2 (or a linux server, etc.. but for windows, LTSC IoT is the most lightweight and secure, 21h2 brings native linux support) natively, here's a quick blitz-through on how to do it without the nvidia M79 chipset having a stronk [if not you can stop here, enjoy catalina and the rest of your day :) ]:

idk if this fixes ALL the crashes (some wifi cards crash i think? nvidia drivers DO crash) but to run native windows (not tried linux or bootcamped tbh, but bootcamp nerfs windows performance on purpose i hear) you have to install via the Legacy Boot, so either via DVD burnt as legacy bootable (not uefi/efi, or csm) if it fits, or the method that i found works for USB sticks (usb won't be noticed unless it's an efi boot) have one stick to boot into rEFInd, and use that to boot into another stick formatted to fat32 legacy boot mode (i used YUMI to achieve this as Rufus didn't work, YMML for linux and grub loader) and you should be good, but as an extra step it might be worth formatting your ssd for mbr first (WARNING: this step wipes ALL data on the disk):

(idk how for linux, but) on the windows installer once it's loaded to the first screen, hold shift and press f10 to bring up CMD > diskpart > list disk > select disk 0 [at least it was always disk 0 for me, disk 1 was the 8gb rEFInd usb stick] > clean > convert mbr > exit > install windows. might be unnecessary, but it's a peace-of-mind thing for me

the x64 windows drivers are in bootcamp 4.0.433 on archive.org (you may want to run the Bootcamp64.msi in compatability mode from an elevated CMD to get it to install everything, and then backup the drivers your system actually has, and then reinstall windows again and device manager update using the backups... personally i could only just not get the webcam working without that but since you plan on doing a server just disable it and install what you need lol. make sure you install the nvidia drivers from nvidia's website BEFORE connecting to the internet at all, just cause windows update will install an old one, and the bootcamp's one is so old it doesn't even work). the first thing i'd do after getting into windows is to install the nvidia co-processor driver and reboot (that's the chipset driver). and voila! just run o&oshutup and windows10debloaterGUI.ps1 to get rid of the few dreggs of bloatware and windows' telemetry, get Waterfox (cause &&^& anything chromium) with uBlock Origin, and you too can laugh when your friends and family's jaws drop on seeing how an """"obsolete""""" 13 year old imac is just as fast as their 2 year old laptop with 4 times as many cpu threads and twice as fast ram! ;)