When I finally had the $$ to go and buy my 'final' trading ship, I looked for something that could;
Fly thru asteroid fields relativly easily
Had good fields of fire for all turrets
Had a fairly solid hull
Was reasonably maneuverable (for a transport)
and came up with the Firefly.
I then went and bought the best turrets I could find, an armor upgrade and eventually (when I realised they were available) the toughest Transport sheild I could get my hands on.
The pity of it is, despite all of the goodies I went out of my way to get, I am hard pressed to fight off even ONE pirate in a Fighter and just forget about anything more powerful, I just stop, hand over the readies and hope they don't kill me after.
I'm pretty sure every trader out has a similar story to tell.