Personally I wouldn't go for any Gunboats at all... They're so easy to kill.
Since you guys are a bit chummy with the Outcasts, I guess the IMG Gunboat is off the table as well?
I'd suggest getting individual Gunboats from factions you've worked with in the past. I don't think there's going to be any 'open market' Gunboats for you guys unfortunately. Maybe we should make a non-outcast Border World Gunboat. I'd be happy to give it a shot, it would fill a nice niche.
But it really seems like you guys have diplomicised yourselves into a corner...