@uireef, I'd go with the Super Marin 250 without a doubt on that size tank. If you're pushing 500g or over, then go with the Super Marin 300.
@sportzfish, There are only two different models of motorblocks used in the Royal-Exclusiv skimmer line:
Askoll 1500 - BK Mini, Vertex Alpha, Slave pump on SM 300.
Askoll 2000 - All Super Marin and Delux skimmers.
Then the differences are volute sizes, venturi sizes, impeller head sizes, etc. Any Askoll 1500 will fit your Gen 2 Mini 180, but the problem is that the pump plate that the block is glued to is specific to your Gen 2 Mini 180 only, so if you pick up a 1500 block for another skimmer you'll have to use your existing pump plate.
@sportzfish, There are only two different models of motorblocks used in the Royal-Exclusiv skimmer line:
Askoll 1500 - BK Mini, Vertex Alpha, Slave pump on SM 300.
Askoll 2000 - All Super Marin and Delux skimmers.
Then the differences are volute sizes, venturi sizes, impeller head sizes, etc. Any Askoll 1500 will fit your Gen 2 Mini 180, but the problem is that the pump plate that the block is glued to is specific to your Gen 2 Mini 180 only, so if you pick up a 1500 block for another skimmer you'll have to use your existing pump plate.