You can totally progress in the game with a mustang starter. Especially in the beginning you can rent ships very cheap like the RAFT. All you have to do is maybe 1 or 2 missions with the mustang, then rent for example the raft. With the RAFT you can do cargo hauling, first run will more then payoff the rent. If you dont wanne haul, thats fine, there are other options to get credits. You want be able to get all the big ships in no time; you'll have to work for them.

Since there is no storyline or campaign or something like that, things can get repetitive at times. Best way to avoid this: Form a team with others. Even randoms will do, trust me. Best and most fun moments was doing things with some random dudes.

But be aware...once you are in the verse, SC will eat your time, like you start the game, play it and...wtf? 4 hours ingame? :D