You can use the old weapon teams. Just put them base to base. Remember that the loader is basically a wound marker so sholdn't count as far as number of hits from templates and blasts etc. You could also buy some 50mm(??) bases and mount them on those. I'd recommend the thin wooden laser cut type so they are thin and you could simply glue your guys on without removing their bases and they wouldnt be too tall.. (then you could add scenic material to make it look good.)
One of the recent White Dwarfs has the stats and points for all the assassins, grab that instead of buying the assassins codex right away. I don't know the number, but it has pictures of assassins on the front :)
I wouldnt bother with buying anything new for now. Your Kasrkin can deep strike without a Valkyrie for example. Here are things I like to use with my IG (drawing from your list of stuff available):
1-2 Company Command Squads, with 4 plasma guns each. I keep these behind the lines of normal guardsmen so they have a cover save. Then I use the Fire on my Target order to make them ignore cover when firing through my own units.
2-3 PLATOONS, each with a platoon command squad (4 flamers) and 2+ infantry squads. I arm my infantry squads with either flamers or NO upgrades at all. (I miss the days of the lasrifle sergeant.)
1 of the platoons will have 4-5 infantry squads so I can form it into a "Blob squad".
1 platoon will typically have a heavy weapons squad with 3 lascannons, which will stay behind th lines and be given the Fire on My Target or Bring it Down orders.
This is my core force. I usually go with either no vehicles or a Tank Commander unit of 3 tanks. If you add Creed to this force he can really help with all this infantry.
I said I wasn't going to recommend buying anything, but there are actually a few things I would recommend buying or converting:
1+ Inquisitors: These can make your blob squad Stubborn with Ld10, they can have psychic powers (Divination in particular), and can have several special wargear items that are usefule. I like to bring one with Divination and the Psyocculum. I join him to the lascannons or a blob squad and they have BS10 vs psykers (and their units). (This is really brutal with the blob squad of 50 lasguns using First Rank Second Rank fire).
1+ Priests. If you use the blob squad I would seriously recommend 1 or more priests be added to it. This will make them fearless and give them hatred. Fearless is the most important thing.
And as nsolomon99 mentioned, the IG have some of the best superheavies available. My favorite superheavy tank is the Stormsword (IIRC). This is the tank with a 10" S10 ignores cover blast. My opponents hate and fear this tank. Just make sure you surround it with infantry to prevent deep strikers from killing it on turn 1 and 2.