Your sergeant is pointless.
When you attack in combat you choose a weapon to attack with, so your sergeant would either
A) attack with his power fist, 1 attack basic + 1 if vetran + 1 if charging, hitting at strength 8 and ap2
B) attack with his lightningclaw, 1 attack basic + 1 if vetran + 1 if charging, hitting at strength 4 and ap3 and rerolling to wound
C) attack with his melta bombs once
Powerfists and lightning claws are both specalist weapons, this means you only get +1 attack for 2 close combat weapons if you have 2 of the same weapon.
A power fist and melta bomb are both strength 8, most vehicles are armour rear armour 10 (you always hit rear armour in combat) so a powerfist is just as likely to destroy a tank as melta bombs and has more attacks. having both is a waste of points.
You have 2 flamers, these deny the marines holding them an extra attack in combat which contradicts the two expensive combat characters in the unit.
Ask yourself what is this unit for? Will they be combat or shooting?
An example of what I mean - I have 10 stearnguard in a drop pod, they destroy monstrous creatures and high armour units, 20 shots always wounding on 2+ creates lots of saves on expensive things that are hard to kill. I could give them meata bombs and have them kill tanks but its a waste of points because that's not what there there to do.
As for how to get bits there are several ways depending on your situation.
A) Ask others. It's impossible to use all the bits in a box, I've tried. Ask other marine players at your club or the shop you play at, chances are they need bits too.
B) eBay. For one off bits eBay is great, people make a living out of opening boxes and selling each piece separately. It can get expensive if you want a lot so sometimes it's worth buying the box yourself for the spares.
I wanted £28 of pieces from the sternguard box so decided to buy the £35 box as id get it all, I easily got £7 of trades from the rest.
C) Upgrade spues / boxes. sternguard have 2 of every special and combi weapon, I believe the vanguard have 2 of every combat weapon and devastators have 2 of ever heavy weapon. Biting the bullet and buying the unit can set you up so you don't have this issue in the future.