For me the most desirable OXP's are not to do with eye-candy. Nor with allowing me to gather billions of credits.
For me the main purpose of the game (as with the original Elite) was to become just that, i.e. Elite. And to become that you need 'only'
6400 kills. And as befitting the name of the original game, that is a tall order. There's quite ( 
) a bit of combat going to be involved.And without using silly OXP's like killit it will be a long and drawn out slog. So I am going to need most of all little things happening along the way. Little improvements to give a change from the daily routine of flying from A to B, dropping off cargo, getting some more, all the while keeping a lookout for food for my lasers.
Although GN and RH can skewer the economics of the game, they also provide you with an ability to do something else for a change, like beating a whole invading Thargoid fleet 'almost' on your own
, taking on enemy beattleships in your little trader/freighter/fighter. And stand a decent chance of surviving. (That is if your tactics are any good). If not ... well, you get another chance if you screw up so you can try something else. 
So for me those two OXP's are a definite certainty. I am not going to install them just yet, mind you. I tend to wait until I have assembled a decent ship first.
Secondly a different ship gives you a different type of game. For me Cobbies are excellent to begin with, even though you need to make sure you have a clean pair of underpants for every one of the first ten to twenty jumps. Just because this is the baddies chance of stopping you before you get strong enough to stop them. And they will go for it. So thrills abound in the beginning of the game, with a few lucky escapes and a few Hooorrrraaaaaaaaaaaaah's!
From personal experience, be careful though. The mizzes ('Ur that shall be obeyed at all times) will not understand it when you shout at the screen at the top of your voice: "Eat this, you blue furry GIT! Who's the sucker now, ehhhh?!"
I agree that you can 'weasel' out of this in the beginning by flying to the sun first before heading to the station so you will miss most of the baddies along the way. But that is not my style.
But before long, the Cobbie will show it's main weakness, a not too big cargohold meaning that the credits you can gather per run will be limited. So I will get some OXP's that allow me an almost natural progression, with an STE to come after the Cobbie for the additional cargo, then wait until I have enough credits to get me a Python and then we are really making a few bucks. There is nothing so nice as to take en almost empty Python out to a planet, fight a few baddies along the way and scoop up their spilt goodies and sell them for a decent buck. For that, and to stay 'true' to the original game, I use the Illegal goods OXP. Plus it comes up with a few sub-missions along the way so another distraction.
Of course, the Python has a drawback too. The first is its speed which means you will be eating the dust of nearly every other vessel en route to the station. By the time that you are really getting upset about being the snail, the big hold should allow you to convert to a Python Special ET (another OXP), with still 100 ton cargo but pretty decent speed.
I'm not doing the starting mission OXP's anymore. Done that, got the teeshirt, time to move on. But for a new commander they are a perfect step into what sets this game apart from it's predecessor ELITE. And that is the OXP. Because of these extensions, you can build the game just as YOU like it.
There are oodles of OXP's that can help you, like the IFF police scanner to see who is bad before they start taking pot-shots at you. Or the extra fuel-tanks, so you can refill your tanks with 3 LY of fuel, VERY handy when you have overused your injectors, and so on, and so on. The other players have given a list of some of them already but I would suggest to go to the wiki and check out the OXP's that are listed there. It is well laid out so you can pick things up quite quickly. It is important that YOU can work out which OXP is going to help you with the way you want to play the game, which OXP's won't really help and which are going to make life more difficult for you.
If you don't particularly want to be a trader, then go for a fighter ship, there are plenty about. Or go for an ultimate trader. of course the nice thing is that if you install an OXP, use it and find out you don't like it, you can very quickly get rid of it again.
So I can't really say you need to get this OXP and this OXP but not to bother with the other ones. That all depends on you and your style of play.
But because of all the hard work done by the OXP-guru's (Stand up and take a bow, guys, you DESERVE it.) it is quite easy to create a game that you are happy to play and will keep on playing. And keep on playing. And keep on playing.
.
