Cats (caterpillar tractors) are workers for a specific job. Hauling cargo is not one of them. And most the time I see a Cat on the highway its sitting on the flatbed of a truck.
Now this is what I imagine the Drayman would be in real life
http://www.bobyoungprints.com/images/autolrgMac truck .jpg and with the correct upgrades could eiter be something like this
http://mailer.fsu.edu/~akirk/tanks/UnitedStates/armored-halftracks/usaht-T12-1.jpg or this
http://www.umzugs.com/pop_up.htm but not both.
What we really need is a ship constrution guide. Some standard for upgrading ships. I like the limited number of spaces for upgrades, but I think that number should be small enough to accualy mean something. Being able to buy the
best of every part and max out your ship should be impossable.
Here is a way I would do it.
Low tech, poor quality parts = 1 space, cheap
Low tech, medium qulity parts = 2 spaces, inexpensive
Low tech, high qulity parts = 3 spaces, average price
Medium tech, medium quality parts = 1 space, average price
Medium tech, high quality parts = 2 spaces, expensive price
High tech, high quality parts = 1 space, 1 space very expensive
External or bolt on mods have a limited amount / ship and cost engine performance instead of spaces.
Tarsus can only mount low tech parts nativly. Modifications are required to mount medium or high tech parts and cost 1 and 2 extra spaces. Extra spaces can be exchanged for lessened cargo space (and a substantial amount of credits). 20 units of cargo = 1 space. Or in the case of bolt on mofifications (turrets, extra cargo, fule pods, etc...) they would be limited to X bolt on spaces max (non negosable) and reduce your engine performance (due to extra mass).
The Demon on the other hand would have less options for upgrade (it was only designed as a fighter, not a general purpose space frame) but can mount all high tech equipment.
Possable new equipment could include:
Fule tank: Bolt on mod. Reduce performance same as adding cargo expantions.
Turret: Bolt on mod. Cost 1 space (for the internal mechanisioms) but adds 1 space for weapons. Half the manuver cost of cargo expantion.
Heavy Turret: Bolt on mod. Cost 1 internal space, adds 2 spaces for weapons. Same manuver cost of cargo expantion
Fake Turret: Would show on scans to be a real turret (to diswade pirate AI from attacking, or smuggling) but not cost any internal space, but only provides half the cargo space of a normal cargo expantion. Manuver costs of a cargo expantion
Smuggling compartments: Cost one space and allow X amount of cargo (subtracted from your total amount of cargo not to exceed 50%) to not be scanable. Low quality would be scanable by high tech scanners but is cheap. High quality is completly immunte from scans but is very expensive.
Ill post more when I think of it.
And back in my day our Tarsus's were peddle powered. And that was top of the line. Gravety wells were everywhere and we had to hire crews to get out and push. And we liked it.