That's actually a very nice used BugEye. It's one that needs good brakes and tires right away as you point out, but the other needs can be addressed while enjoying it this summer. The Datsun 5 speed is a popular transmission replacement. I recently rode in a lovely Sunbeam Alpine with that transmission switch.
does the date box require bodywork modifications to fit? I've got a Datsun 210 5 speed and 1275 engine, ideally want to fit the data to my Bugeye, but don't want to cut the frame
I imagine another good gearbox for these would be an early Celica one. Those are legendary, capable of handling a lot more power than the Celica had, not that a Sprite engine produces anywhere near the power to push it :)