Sorry its taken me a while to reply. I have been painting all day.
On scouts I would go with a combi melta before giving them a powerfist. Powerfists work better on tactical squads, where the unit can generally hang around in close combat.
I like the classic destructor preds too, I own two, including a heavily modified mk1 plastic one. Lascannons may well be expensive, but they are only around the same price as a devastator squad with them. A single autocannon on its own is vunerable to weapon destroyed results.
Aldaris is talking about the scout melta torpedo. A scout squad in a speeder storm, give the sergent a combi melta and power fist, the speeder has a multi melta. You scout before the first turn, then charge an enemy tank in your first turn, giving it a blast from both melta weapons on the way. You will then die since your unit is on its own right in the middle of the enemy army. So you need to find an enemy unit either very expensive, or very useful to the enemy to make it worthwhile. Pretty much only Land Raiders and Monoliths qualify for tanks, but if you can waste an enemy lone HQ or something similar, thats also good.
If you decide to have a scout squad which stands off, four sniper rifles and a heavy weapon is a good idea. They out range a lot of things due to the high range on the rifles and heavy weapon. Sure the BS isn't great, but the missile launcher and heavy bolter are okay with this bs.
Regarding the third squad, I would go with either a multi-melta or another missile launcher for the squad. Now I could suggest plasmacannon, because they are my favourite, but they are not the most efficient weapons! I just don't think marines really need heavy bolters, as bolters are more than capable of working as anti infantry weapons.
