Phil,
The fuel purifiers and scoops make roughly zero difference to the economics of the ship - they are within the 1% "slop" thats gone to self-defense and that doesnt really change the economics of the ship at all (plus here for Andrew Vallance's High Guard ship creation program).
Yes, you have reservations about whether this ship is required, because you dont understand just how much volume of stuff worlds produce - lets take, for example, the Tasmanian beef industry.
Lets assume that, somewhere, there are people with a taste for Tasmanain beef, and are willing to pay Imperial Credits to get it. One imperial credit per kilo, in fact. Call it 1000 kilos per m3, and call it KCr10 per dton.
At least per FFS2, it's easily going to be cheaper to import meat at a Cr1 per kilo than it is to upgrade your life support system to produce meat. With CT, it's *handwave* *handwave* "life support costs" *handwave*.
Now,Tasmanian cattle farmers are currently being paid roughly $4000/ton for their beef ... in local currency, which doesnt buy starships, hitech weaponery and all the other things a TL8 culture wants. Trust me, in Imperial Credits, you'd be paying Cr400 a ton, maximum, or KCr 4 a dton (and check the CT-canon exchange rates in Trillion Credit Squadron or Striker for some not-bad numbers).
Tasmania exported 24 kilotons of beef last year, plus 7 kilotons of sheep meat. Thats 31 kton, or call it 3100 dtons. Lets see, a 200dton Far Trader a week, moving 80 dtons or so, moves that with some space left over.
On the other hand, Australia exported 1.4 megatons, or 140 000 dtons.
Remember, this is just beef and sheep meat going from an Ag planet to non-Ag planets. No minerals. No booze, No grain. Nothing else.
From what we know of trade patterns now, just the meat trade, going from an Ag world to various worlds without biospheres, at a target price to the customer of ICr1 per kilo, will be enough to support multi-kiloton cargo ships.