I've been longing for a playable grim reaper mod that isn't just a costume but full on traits and powers with the works

if there already is a mod out there that lets me do such a thing point me to it

Note: I cannot afford sims 4 so don't suggest it please

Oh dear, I would never suggest Sims 4 to anyone!
You might want to post in the Sims 3 WCIF forum, since you are asking where to find an existing mod rather than how to make one.
https://modthesims.info/f/559/

Well I am willing to learn how to make one myself to be honest as long as it isn't too difficult to manage

I'm not sure how difficult it would be, I guess it will depend on what you want to do exactly but it will involve script modding at least. So if you want to try some of the script modding tutorials (here) you could start with the object modding tutorial to get the hang off making custom interactions, and the pure script modding ones, like adding interactions to sims etc. Then once you get the basics going, you can plan out what exactly you want from this mod, and of course post here if you need some help with getting started