Hi Paul
A 300mm focal length prime lens (with a full aperture solar filter) should cover the entire disc according to Ron Wodaski’s fabulous and free
CCD Calculator. However, it might be prudent to reduce the focal length to say, 200-250mm to allow sufficient empty space around the solar disc for alignment, tracking and aesthetic reasons, so a zoom lens may be the way to go?
You’ll also need an adapter to convert the lens thread/bayonet to a 1 ¼ inch fitting to accept the ToUcam.
You can practice on the full Moon as the angular size of the Sun and full Moon is almost identical.
Cheers
Dennis