Normally, after having a great, small, normal, fast, light lens, we can look at both extremes, to cover the less usual...
I agree with mfogiel... 35 for one lens only, but for two lenses 28 and 50. For 3 lenses? IMO 28 has a true wide character, and a real tele is needed, so the center one should be a normal... Then, 28, 50 and a real tele...
OK, you said 21 and 35 are coming, so do I pick 75 or 90? We give options, and opinions... All this is not just for the OP, but for other members and for the future...
As I see it, and some others too, there's a gap between 35 and 75/90, and it's a critical gap, because it includes normal lenses... If we try to make that gap smaller, preferring with the 35 the 75 over the 90, we get less tele power apart from losing the normal view yet... 2 problems instead of 1...
So, a light 90 is great for isolating urban scenes and landscapes, not to mention portraits: it can see a lot of use, while a 21 isn't really a good choice: don't get me wrong, I own 3 different focal length lenses wider than 21, but it isn't really a good choice for a very few lenses set... The 15 Heliar is great and small too, and means lots of fun for sure, but those reasons are not enough to include it in a truly basic set...
In the end, you´ll take your decision by yourself, basically... We all wish you a great trip, and enjoy your M9 and a few small lenses...
Cheers,
Juan