None of my regular players were ever interested in playing free traders. They had a pronounced aversion to accountancy. So my Traveller campaigns were planetbound with transportation provided by the patron or, in one case, a naval campign where the players ran three characters each, an officer, a crewman, and a marine, all part of the same ship's company. Ship paid for by the Imperial Navy. One quite successful campaign had the PCs be troubleshooters for Oberlindes Lines. Several adventures became short adventures or amber zones for JTAS Online.
I did try to set up a merchant campaign -- twice. The PCs were the officers on an old, obsolescent 400T unstreamlined
Golden Gryphon Class J2 Merchant belonging to a fledgling line (Uakye Transport Partners) and used by the company to "fill in the cracks" when their regular ships couldn't deal with it. The first campaign was very enjoyable and lasted for over a year, but since it was conducted online, we didn't get very far in terms of game-time. Over the course of the campaign the
Silver Sphinx managed one return trip from Regina to Ruie. I tried reviving the campaign a couple of years ago on the Traveller wiki. I gave all the former PCs food poisioning and had the new PCs recruited in haste to replace them. I had the impression that people were enjoying themselves -- I certainly was -- but for some reason it petered out fairly quickly.
So I've never had to find the ideal adventuring starship, but that doesn't prevent me form having opinions

. The ideal adventuring ship is old worn and bank payments are much lower than for a new ship; 25% of new price, perhaps. In return, the crew has to save up to pay for repairs when the inevitable breakdown of a power plant or drive or life support system or whatever happens. PCs can afford to spend a few weeks on a planet doing something else without insurmountable debts piling up. Indeed, when something breaks they're forced to spend time looking for other sources of revenue than trading.
The ship has a crew of about 8 or 9. Enough for 4 or 5 player characters and enough NPCs to leave behind to hold the fort when the PCs all go gallivanting off on adventure (A henchman is one of the most useful pieces of adventuring gear there is).
The ship has at least J2; J3 is even better.
The ship has enough cabins for about a dozen high, medium, and economy passengers; it may or may not have low berths. Passengers are a rich source of drama and exitement and plot leads.
Hans