For the auxiliary grenade option, you'll want to take one per 5 guys, as it's free and a great upgrade. Always do it.
For the bolters, you have three options.
Stalker bolter can be very usful if you want to stay mobile. The lack of ap can be mitigated by the one warlord trait (forget the name).
The stalker bolter I'd recommend this for objective holders. Maybe a 10 man squad you can split.
For the most part, the standard bolt rifle is most universally useful. It has great range which is always nice.
I think if you don't know what role you want, the boltrifle is the best bet.
If You're using white Scars chapter trait, I'd recommend assault bolt rifles, as they benefit a lot for being able to advance.
Stalkers are fine, but they really encourage you to keep the unit stationary, so if you are taking a couple of small squads to hold objectives,that's not a terrible choice.
In chapter approved, you'll be able to add power fists to your squad leaders, which could be fun.