Hey paralaje, I'm still pretty much a beginner so here's some concise advice and explanations from one beginner to another:
First off, your specific questions have more or less been addressed.
Second, like it's been said, you're wrong to go for the first planet you see. The planet in your system is a pity planet at only 4, it's not high on your priority list. Select a colony ship, and look at the minimap. make sure you can see stars. You'll be able to see the range you can go and when you turn on the 'ownership' highlights, you can see where other races are (and where they'd likely expand) Generally, you'll want to find and capture the nicest juiciest planets as quickly as you can. Not only do you get bonuses for planet quality, you can build many more buildings.
As for economy buildings, the basic ecomnomy works like this:
You get money mainly from taxes. The more people on a planet, the more taxes you get. The more economic buildings on a planet (such as trade centers etc) the more money you get from tax collection. So you can see how having a slow reproduction rate/low number of initial colonists don't generat e alot of moolah, there simply aren't a lot of people to tax.
Factories build things, like ships and buildings. The more factories you have, the quicker it can do so. BUT, theose factories can only operate at the level you set the slider to in the economic screen. So you can have tons of factories but they won't do a thing if they don't receive money.
Research buildings increase research, but again it depends how much money you give them. Just remember that research and factories are essentially money gobblers, you get money from taxes and the more people on a planet the more taxes you get.
here's what I do specifically at the start of the game:
General rule: get as many planets and minable resources (those little colourful dots) as fast as you can.
- Set my flagship to autosurvey (the money it can find is neccessary as there's always an economic dropoff)
- set production to 100%, and for the sliders I usually bring research down a bit so planetary improvements and ships can be built quicker (that and reseach is hell on money)
- Rush buy a factory and a scout ship on my main planet. The factory helps build a few more factories, then a mix of economy buildings and research centres. It also helps with the speed that new colony ships and constructors are spat out. The scout is relatively cheap and can help me be more efficient in actually finding nice planets.
- top off the colony ship and send it to explore towards the edge of my range, heading to other empires, checking on stars along its path for nice worlds. the reson I head to other empires is that they're probably going to expand along my way, taking up some nice systems.
- I work from outwards in, taking planets at the fringe of my range and then eventually working on getting the interior and less valuable planets. Since it will take a longer time for the enemy to reach into my space, so I try to head them off at the pass.
- When building on a planet, the first few nice ones I find are economic based. I build economic buildins, as well as approval buildings (if the aproval is higher the reproduction rate will increase) I've also taken to creating influence buildings on my eco planets, as I thought I read more planetary influence increases tourism. Tourism itself is important economically.
- I try to find 3 good quality worlds for each of my capitals: production capitals either have tons of spaces or have tiles with manufacturing bonuses and are closer to the galactic center. Research and economic capitals can be anywhere, as long as the economic one has more tiles. I also might have a few more manufacturing planets (I play more of a conquer them after I researched incredible tech, so I don't need a lot of manufacturing areas) Manufacturing takes up money whether your building something or not, so if you have a lot of factories or research centers on a planet it probably won't do well economically. The general rule is make more economy based planets then the others.
- I wait to trade. It's more important to get as many colony ships and contructors out there to get plents and take and upgrade mining bases. Only after I have everything I can get without force I start building freighters.
- From there it's waiting until the economy gets better, then... whatever you wish. I usually try to get a head start in research, taking a few weeks off to spurt out a few high-tech war machines to deter invasion.
Anyways I hope all that helped (I hope it's at least somewhat accurate too)