Mundane Wagon Enlarging and Modification?

[WHAPSquad]

So, I've got a bit of gold to throw around at character creation (7,580gp to be exact, at level 3 no less) and plan on creating a caravan to fight for me, while I do the trading, talking, and all that good stuff without too much magic.

To the point, is there any 3.5e rules for making larger wagons, adding specialized weapons to them, and making it livable in them.

The one I'm having the most difficulty with is thinking of a way to make a sort of small "Potion Cannon" for my to-be alchemist's wagon, one where she can put some Alchemist's Fire in there and blast it all without having to step outside. I don't know much about artificing so I'm not sure how much it would cost to get that built if at all possible.

[ShurikVch]

If you're OK with 3.0 rather than 3.5, then Arms and Equipment Guide have some

Firstly, Masterwork Vehicles: +5' speed, and +5 hp per inch of thickness

Then - Mundane Augmentations:
Armor Plating
Ballista
Catapult, Heavy
Catapult, Light
Firing Castle
Masterwork Controls
Passenger Spaces
Passenger Spaces, Fancy
Passenger Spaces, Luxury
Ramming Prow/Plate
Rotating Platform

[Maat Mons]

If you're going to be living in a wagon, I highly recommend laying an Enveloping Pit on the floor. Or paying someone Lawful Neutral to do it, if you don't have the right alignment to lay it out yourself. Either way, you now have a 50-foot deep extradimensional space in your wagon for the relatively affordable price of 3,600 gp. Build a tower in it, and then permanently attach the top of the tower to the inside of the wagon, such that you can't even see the edges of the Enveloping Pit.

The halfling warwagon and mobile redoubt from Arms and Equipment Guide are both enclosed and armored. Potentially hand if you plan to fight from inside them.

Speedy Wheels, also from Arms and Equipment Guide, increase the speed of the vehicle you attach them to by +10 feet. The price is a very low 100 gp, though I'm not sure if that's per wheel or per pair of wheels. You need a pair to get the benefits. But you don't need more than 2 wheels (1 pair) no matter how many axles the vehicle has.