What crankset are you looking at? you can get 3 chainrings, for about £40-£50.
I need to replace the drivetrain (sprockets chain etc) and can get a crankset with a bottom bracket for not a lot more than the cost of the front chainrings and just saw different options available so was wondering if it would be worth changing things
My racing mountain bicycle had a low bottom bracket and 180mm crankset and I never hit anything.
Go ahead and try some 180mm cranks and see if you like them. If not try some other length.