'Niederschrift' is best rendered as 'memorialization.' That's when parties put pen to paper and mean to be bound by the formal document. Here, a list of specially purchased items will be annexed to it.
This final document might well serve as the customer's acceptance, too.
'Memorandum of agreement' doesn't work because it's usually the tool one employs at the start, not the end. Here, the parties agreed that the developer would create a customized system or product of some kind based on its perceived needs going in. They obviously change along the way once the expert gets in there and sees what's required.