Hi, i'd like to make a pc case but with no light leaks or smell whatsoever.
Possible, but that's up to you.
Is there a way to block the light coming from the fans.
Of course. Just build light traps.
The idea is just to have some piece of pipe or box so that air can flow through easily but there is no straight line path for light to get through.
The simplest design is just a piece of pipe with a right angle and the inside painted black (to minimize reflection/glare). You point it away from the light source.
A more elegant (and potentially smaller) way to go is to build what amounts to a small box with baffles inside.
I'll also note that with a typical PC case light will leak out OTHER places too (eg the side vents, around the disk drives, out the back where the wires attach, etc), so you have to be diligent.
Would I be able to put a carbon filter on the outtake fan which would be at the back and intake at the front.
I've never seen a commercially available carbon filter that would be suitable for a single PC fan. Most of them are roughly the size of a 2-liter soft drink bottle and are designed to run with 4 or 6 inch ducting to exhaust a closet, grow tent, or even small room.
I don't think a PC fan moves enough air to push through one, and even if it did, the filter is so large that if it were on the outside of your computer case you'd have no stealth. On the inside, you'd have no room to grow anything.
You'll have to either build a custom carbon filter (you can find plans on the internet), or use some other method of odor control.
Here I'd note that smell is proportionate to flower mass (ie amount of bud). Even a small plant definitely will generate some smell, but probably not that much from one tiny enough to fit in a PC case. You might be able to reduce it with an external ionizer or other air fresheners in the room.
