How much storage space do I need to work with this 750GB project?

A few more thoughts:

1) Which some a large amount of media, you should keep it external. If it is already in one of your drives, import using Leave in Place. You do not want to make unnecessary copies of all that. But do keep BACKUPS! Your 5TB drive is great for that.

2) Do not create proxy or optimized media either, unless it is really necessary. That can take a lot more space than the originals.

3) Turn off background rendering and only render, manually, sections that need it for smooth playback.

All of the above will contribute to avoid an explosion of disk space usage.

Using external media, the library should be quite small and could easily be stored in an internal drive.

Also, using external media, it can be stored in more than one drive, if necessary.

This means that perhaps you don't have to have a large and expensive drive.

You could start with a 1TB drive, and add more if necessary.

There are plenty of options. I have two of the Crucial X8. They worked great for me with a USB3 connection, and even better now that I have a mac with USB-C. They are USB-C drives and come with the proper adapter to connect to a traditional USB type A port like your mac has.

Of course there are other drives out there, I mention these because that is what I have and they serve me well.

I suggest you go with an external drive. This will still be much faster than an HD, and I doubt that you'd see much benefit from using an internal. Although theoretically faster, at some point the bottleneck becomes the human, and not the machine.

Plus, with external drives, you gain more flexibility. Maybe you need more than one drive, and that is far more manageable externally. Plus, your mac is fairly old now, and your new SSD may be later used on a shiny M1 Mini, for example.