UH1, it would be interesting, but a campaign has already been released.
Mi8, there are several campaigns.
Gazelle, there are no campaigns, but focusing on a campaign seems like a slightly counterproductive move.
I would start working on a Kiowa campaign: D
Trying to make one (1 single campaign!) For the gazelle could be a test, but it's better to wait for more important updates, especially for the poor flight model.
At this point, at present, follow the following lineup
1) Mi8
2) UH1 (if the first campaign has had a sales success, I would do only one in the civil sphere, no military role)
3) work in the background and collect material for a future Kiowa campaign
4) collect material and plan a gazelle campaign (but only after major updates)
ps, I have never bought any campaign, because I always fly only in multiplayer.