Hi Amon2501. Shields and shield boosters all add to your shield strength. I don't think there's any technical reason why you can't buy the Military Shield Enhancement before you buy the standard Shield Boosters: it's more of a budget issue for most people.
Buying a different ship will give you different capabilities. The Cobra III is a good all-round ship, with decent speed, cargo space, armaments and manoeuvrability, but there are obviously ships out there which are better in some (or, in the case of some OXP ships, all) areas. The Boa Cruiser is tough, and a good fighting ship - but one thing to remember is that it is larger than the Cobra III, and bigger targets get hit more often!
In the vanilla game, some ships don't work very well - for example, many of the smaller and older ships are not fast enough to overhaul other ships and escape from mass lock, so you have to navigate around traffic if you want to use your Torus drive. Which is not to say that these ships are wholly without charm: flying an Adder, especially in combat, is a real experience … just watch your energy levels.
The main difference between Rich and Poor systems is the price of goods. The most advantageous price differentials are to be found between Poor Agricultural and Rich Industrial worlds. Like Mr Gimlet
Rich Industrials are rich because they’ve got the most efficient processes, so not only do they make the cheapest products, their factories are the hungriest and they’ll pay the best prices for raw materials. Poor Agriculturals, on the other hand, they’re most desperate for fine articles and will scrape together whatever they can to pay for ’em: meanwhile, they’ll offer you the cheapest deals anywhere for what they make themselves. Which puts a vicious lock on the poverty trap, but hey: nobody said life was fair.