Fine, but only up to a point. Equipment from E. Europe, all made up and ready to go, is cheaper for me to buy than I can get the timber for in boards here. Never mind valuing the labour or even having the time to make my own.....
Also there was a slight gripe somewhere in the thread about members not sharing information......well many do, myself included, but there is only so many times you can give advice and have it contradicted by people who say they can do this or that cheaper......in ways I would just call pottering about or even scavenging.... before you feel a little ground down by it. A lot of good posters just get on with life rather than get 'into it' with two hive two season beginners who know better.
DN4? Why? DN5 gives less scraping and cleaning to do. Once you become a bee farmer something that takes even 30 sec per hive per visit adds up to a significant cost over the season, and if you don't cost it its still a hobby. No text book from the UK I have seen tells you another thing. Different spacing in the brood area and in the supers causes obstruction of the vertical airways inside the hive. Colonies with the same spacing all the way up tend to be easier to manage and swarm a little less readily than those in the configuration you describe. I have much in the way of heritage equipment that came down from my father and am reluctant to scrap it. but if starting the BS unit again I would be all Hoffman on the same spacing all the way up the hive.
For real flexibility and ability to run with most efficiency, one size units is best.......pick one box size and one frame type and stick to it. Then any box or any frame can do almost any job.
The bad news for the OP is that the expansion plans do not in any way, at this stage, constitute a commercial order. These are sad to say (from the point of view of getting a discount) small orders and a discount of any significance is unlikely. The vendors time is valuable too.