It isn't too early to order, but you don't want to be fitting too soon once you've got it, leave the foundation in its plastic bag to retain its delightful aroma as much as possible. There will still be plenty of time come spring........unless you've got hundreds to do of course
If they are the DN1 SN1 frames I use- can't remember their proper name you can't make up the frames very easily without putting in the foundation at the same time- you will have to leave at least one of the bottom strips of wood off, or you'll be prizing them apart to put the foundation in.