I made a short article as a "First place to look" in terms of what it takes to convert a car to electric.
See that here.
https://300mpg.org/buildelectriccar/
I'll just reinforce an idea or two from the article right here.
See what people have already done. A Miata was always a pretty popular car to convert. Look on the EV Album to see what parts and budgets people had for their conversion. You don't have to re-invent the wheel!
EVAlbum: Search Results
Keep in mind that converting a car is a somewhat long and complicated process. Do it because you like the learning and the process itself (you are a project person, you like working with your hands, etc.) not just for the end result.
There are many more resources today than when I did my first conversion (which was a big reason why I made the videos - they didn't exist otherwise!)
Nowadays, there are quite a few good YouTube videos, Web Forums, and other places to learn. Don't forget about in-person clubs and even your local library!
Again, take a look at my article for more.
https://300mpg.org/buildelectriccar/