You're probably right to be sceptical about your local merchants claims. Now this I would have believed. Homegrown, European and North American hardwoods are easier to source at the moment. Soft wood is ok especially redwood but it will need to be treated which is a problem. Oak or sweet chestnut would make very good and durable windows but don't use american red oak as it is not durable.
Sweet chestnut is as durable as oak and has a lower tannin content and is cheaper.
Reclaimed pitch pine is my favorite. Remember to use coated double glazed units in your windows such as Pilkington K which will save you lots of energy.