|Yep, useful! It removes a significant annoyance.|
Two things would improve it:
1. Say why it can't build the ship. Currently, I see it can't, then scribble down the numbers I need, then have to go back a screen or three to see what resources I actually have. Sometimes I just need to sell supplies, other times I see a deal-breaker.
2. If the parts are there, use them. Although, I can imagine some cases (Feds Super Refit) where that would mess things up. Let us choose. If the default is building new parts, then it won't use parts we've built and carefully set aside, but it risks building more parts than necessary and using up resources. No matter which default you choose, someone will prefer the other.
I suspect #2 will be a lot of work -- you'd have to check part availability rather than just resource totals. Not sure if it would be worth the effort.