Hermes Light Transport

The Hermes Light Transport is one of the oldest ship designs on the space lanes. Some are more than 70 years old and still in service.
The Hermes is attractive to many light shippers and smugglers because, for a brief period during the first war with the Imperial Consortium, the company that made the Hermes converted them into the Mithras Strategic Bomber, which was produced by the thousands and devastated enemy infrastructure. After the war, the Mithras had all the weapon systems removed and were put on the market again as Hermes transports. However, even though the military-grade weapons were taken out, they couldn't remove all of the internal upgrades you need to support military-grade sensors, weapons and speed, and they didn't strip off the additional armor plating.
This means that the Hermes has the additional power, armor and infrastructure to support heavy, heavy modifications.

Also, with so many manufactured during the war, parts are dirt cheap. On the downside, it's old and generally sold unarmed (though they rarely stay that way), and they have the bare minimum of creature comforts.