I'm thinking about invading a planet soon that I've recently found. I was wondering about the requirements to invade a planet. To invade a planet do you need to be a certain rank able to colonize a new planet? Like right now I can't colonize another planet till level 48 - so do I need to reach that rank first? If no - If I invade will I still be able to colonize a new planet at level 48 - or with it take it's spot? Also does it cost credits or anything special to invade?

The amount of planets you can own is limited by your rank. This limit also counts for invading. If you need 48 to colonize a new planet, you need 48 to invade too. Invading takes the same spot as colonizing.
Other notes:
1) It takes energy first to take hit the pop down(10 or less) and get rid of all the defenders.
2) It takes more energy to use the "invade" button compared to the "colonize" button
3) It takes double the amount of credits to invade compared to colonize.
4) Invasion may fail. It is counted by your Attack VS the planet attack+defense+invasion defense.

But at the same time if you invade a world you get all structures that are on it for "free". How ever if they are better structures then you have researched then they will be useless to you and not produce anything. So only invade if it is a great world that is very rich and large or better. Also when you do so be ready for retaliation back at you.

Thanks alot for the help.

if you are attacking your defense and everything but attack and invasion do not count...as for defending a planet im not sure if the defending ship counts for the invasion as its disabled at that point but you would be able to tell by checking the total attack and defense at the colony tab...i think it does though...im not sure

No, for invading, only your attack is counted (as per your attack stat + invasion attack which is available in some reward modules). You do not need certain modules/crew size/cloaking/scan to invade.
While knocking down a planets pop, you will need defense to reduce the damage you take from the planet's return fire.
Cloak, scan I doubt have any use in invasions unless there is critical strikes.

For defending, cloak provides planet with bonus cloak, attack/defense provides bonus attack/defense

i should have known....the defender is gone once you disable him....you have to disable him before you can invade at all anyway...so you attack the planet first and the defenders att/def does count....after the population is down below ten you can then attack the defender....after the defender is disabled, he disappears and the planet is ready for invasion...i hope thats clearer ...bender