I like your set of Models. though I agree with some of the others that Crookedmen and Drowned will really help you, the models you have here will be great.
First off I pretty much always attach the Avatar to Seamus. Making him go from Shooter to hitter is a nasty swap, and he is really strong all around once Manifested. And when you decide to take the Avatar you pretty much need a full cache, so set aside 2 stones for the Avatar, and 6 for the cache. (7 is ok, but try not to go lower)
Now the Crew.
First thing I always want is a Hitter. A-Seamus is a monster in Melee but I always try and drop my opponents best hitter before Seamus pops so he has less competition. From your set I would take Either Bette or a Hanged. A Hanged with a Grave Spirit is really tough, so I might try that Combo out. Either way, set out another 9 Stones.
Next is Seamus Gal's I would grab 2 Rotten Belles, they are tough, fast when he's around. And great for board control.
That right there is 25 Stones. Seamus, Avatar, Grave Spirit, Hanged, 2 Belles.
For 30 I would just add in an Onryo. They can help remove peoples WP immunities which is nice with this list and they are decent offensively.
I strongly recomend getting a set of Crooked men for your next purchase. They can fill a lot of roles for ressers. And are criminally cheap.
Outside of that I would look into a new hitter too. My personal favorite is the Dead Rider, but I think Izamu once released will be rather awesome too
So a few other points to make. #1 He does no need Von Schill. No resser master does. They stand alone perfectly.
Seamus pre-manifest is not a melee guy. He's a shooter and has the best gun in the game. Post Manifest he is a Melee master. But isn't a dedicated Melee Guy like Lady J and the Vicks. He has control aspects to go along with his Melee.
Use your Belles to Drag models into Seamus' pistol range and focus shots at them. Being able to cheat in that 7 damage severe is huge.