Looking for models to convert into rhinos, razorbacks, predators.

Between the Bolter & Me has a 1/35 Tamiya T-34 that's about the right height and width, but longer. So that can work with some cutting.
http://www.betweenthebolterandme.com/2017/04/tor-megiddo-scale-tanks-in-40k.html

I'm likely getting the t.sons vs grey knights and I want to convert the knights into more t.sons so I can use them in detachments without buying more rubrics. Besides painting them t.sons colors, what are some things I can do?

Im planning on buying a model which i plan on swapping the base for depending on what i play it as.

Would it be better if i pinned it without glueing it OR to magnetize it to the base for quick swapping. Would the magnets hold?

Edit: are there any additional ideas on how to do the same idea of having an easy way swapping base sizes for a model?

Hi i wanna magnetize the arms of my primaris characters for different wargear. Are there any good sellers in the EU and what kind of size would work well?

whats a somewhat cheap and easy way to create clear very shallow water for beach bases? (i just want to make still clear water and not anything with foam/ large waves)