It is not necessary to change the expansion tank simply as maintenance.

Unless it is cracked beyond repair and cannot hold pressure.
The white crud is probably calcium buildup. Try to clean it before just buying something new. It's not entirely unusual to have calcium build up in the cooling system. Obviously it would be better without it though.

I would replace the hoses if they are leaking, but if they're not, that is not the source of your missing coolant.