For manual cutting of acrylic it is always wise to scrore the line to be cut first (both sides) with a pointed steel tool or sharp knife
drill a small hole at the extreme of each score line if needed...say if it is not at the edges of the material.
and ,,,the finer the pitch of the saw blade (high tooth density per inch)
hacksaw blade etc ...but cut along the scored lines.
This will help prevent cracking or splintering of the acrylic.
Or invest a fortune and get a laser cutter ..
🤣

what about drilling the screw holes for the mounting to the side panel, as well as the cutout on one of them for the 80mm fan

When drilling acrylic ..I always like to stick masking tape on at the drill point ...
start with a small drill bit say 1 or 2mm ..and work up size wise a couple of mm at a time ..use sharp drills ..
..untill the hole size required is achieved.
(pilot hole)
dont apply to much pressure ..let the drill bit do the work ..
A step drill is usefull tool for this job

To cut a square out of flat plate...drill a start hole in one corner of the square to be removed ..wide enough to insert a junior hacksaw blade ...thread it through the hole ..and reattach the handle if there is room ..
Again ..score lines and drill holes at all four corners _before_ sawing...then follow the score lines with the hacksaw blade.
good luck

Would echo the previous advice.

Techniques I use vary depending on whether I'm using extruded or the more expensive cast sheet acrylic.

If we assume the cheaper (and softer) extruded type...

...for cutting I used to saw with a 32tpi bi-metal hacksaw blade (ideally not the junior type which can over flex), but recently I just score heavily on both sides with a proper Stanley knife, or similar, and snap over a clean edge, then dress to finish.

..for drilling, as said, work up slowly from a pilot hole to the required size having masked / marked the drill point first. You can also melt holes using an old solder tip.

...for fan holes I'd suggest either a Dremel / other decent rotary hand-tool or for the cleanest finish, a hole saw.

...for fixing I tend to avoid screws (nuts & bolt to reduce the chances of cracking) but for preference I don't use either. Although I don't like side fans generally, if you are using fans / grills I'd suggest rubber anti-vibration mounts. For the windows themselves I use 3M / Gorilla heavy duty double-sided adhesive tape.

Also generally, always clamp down the acrylic to a solid (wood) backing surface when working on it and try to leave the protective cover on till you've completed the cuts / drills to protect from surface scuffs / tool slippage.

Well the case is one of the old ones with a handle and a side mount 80mm, UV purple but im going to i think replace it with UV Green and paint the case a solid flat black, with Red LED fans