There are a couple of games, but none of them matches all of the requirements.
Strange Adventures In Outer Space...
... if I rember the title correctly. You are hired by some sort of galactic billionaire and have to explore some area in space off limits to normal folks. Your goal is to bring back as many exciting and valuable artifacts as possible, while trying to survice space monsters, super novae, pirates or hostile aliens. During your journey you can upgrade your ship with the stuff you've found and even hire wingmen to support you fighting the baddies. Trade is limited to trading for artefacts with other races you come across. Each game is limited to 15 minutes realtime, but despite thar short period of time the game is great fun.
Escape Velocity Nova
You play as the hero created by the universe itself, althou not yet knowing. All of the races were once human, but due to some dark fate the colonies were separated and developed independently from each other. Some mastered the art of using the power of their mind to shapeshift into shipy, allowing them to travel through space without the help of anything else. Others made psychical and technical advancements, so they can build ships which don't rely on true newtonian space physics as we know it anymore and have the best equipment for theit ships, but live so isolated it's very hard to make contact with them. It's your choice which party you help to achieve their goals.
Flatspace
Nothing extraordinary, but dynamic trade (iirc) and a lot of exploration.
Space Rangers
Dynamic trade, upgradeable ship, part 2 also has freighter hulls, and extremely difficult to beat. Has planetary quests in the form of text adventures, where you mostly have to solve riddles. In the beginning, cash is hard to come by and best earned by trading. Once you break the 300.000 credits barrier, you can start making missions given to you by the planetary secretary. That's not because of the difficulty level of the missions, but rather because of the limited number of missions which give more pay the more cash you already have. Part two also has planetary RTS quests. A german magazine gave it around 60%. Althou the game is very difficult, that rating IMHO is too low. I'd give it some 80%. It's the best I have played in a long while.
What all titles have in common: 2D and singleplayer only.
There are of course the old classics like Elite, Frontier, Privateer, Solar Winds, Hardwar and newer titles like the "X" series. Alas, I surely forgot to mention half of similar titles. I'll post them as soon as I remember them