The best people to deal with in terms of a max 10 week delivery time for sliding sash timber windows are Bereco in Athlone. We had our windows installed recently and they only asked us for 30% dep of the final amount. We didn't have to pay all the costs until they were fitted, i think it was a 30/50/20% split something like that.
Anyway, dual colour, Gardenia on outside and Walnut interior, they were a dream compared to other issues we've had with building ourselves.
Ours are redwood which is a Scandinavian Pine, same warranties applied as to the hardwood so we choose this at was less expensive.