I think a masterwork stagecouch (a 2 by 4 square carriage pulled by 4 horses; pg57 A&EG), with upgraded to fancy passenger seats would come to look like that rolling cottage on the left in the picture. It would have very high cost though.

The couch comes in at 800 gp, it costs 10 000 gp for making it masterwork, and the cottage is equivalanet to changing the available cargo space (1000 lbs) into regular passenger space for 100 gp.

I guess upgrading all available passenger space to fancy interior would prolong the hours you can travel, so that would be advisable. It becomes very pricy to permit room for that, you triple the price and halve the amount of passengers for a fancy interior. Its times 7,5 and 1/4 of the passengers if you want to travel luxuriously.

So for the party couch I would go for a masterworked fancy stagecouch: 4 light horses, 1 crew and 4 passengers travelling in relative luxury for a mere *12 700* gp.

Added rooftop ballistae are available for a mere 500 gp, get yours afixed to the roof at your loacal dwarven carpenter right now!