“… So could I buy the non-touchscreen versions in the link you gave and it will still work the same with your customisations?” - my customizations should working with any screen with 1920x480 resolutiom (but be carefull while buying - there you need too a board for connecting this screen with RPI - I am not sure if there is a version with this board and without touch 
But in general there should be no difference (regarding to my UI customization) if it is touch or not touch screen.
?..I find that when I edit the CSS it gets overridden when I upgrade Volumio system. Have you found a way to get around that?" - here I can’t help I did not upgraded yet but please read this thread Contemporary optimizations for small landscape displays (7" raspberry) - #112 by lausiv
- there should be answer by 2aCD for your question