Like @file28 says, you can start looking whenever you like. There’s no harm in looking as you could even rule some cars out if they’re not suitable when you seen them up close.
id also keep in mind that between now and then there will three (I think) quarterly updates to the scheme so there’s a chance a car you like could disappear so I wouldn’t set my heart on anything just yet. At the same time new cars could join which tick all the boxes for you.
i hope you manage to find something suitable.