@Stubei
1. No need to grind each planet. Just beat it once.
2. There is no set level you should be. The difficulty gradient is steeper for new, ungeared players. But literally your first Rank 30 weapon loaded with mods should make it an easy challenge. If you're absolutely intent on grinding your WF or weapon to 30 on Mercury, do online missions for Apollodorous. It's where everyone goes with their forma'd warframes or weapons to level them initally, so you're guaranteed to have OP teammates that will help you rake in boatloads of Affinity and Mods.
3. There are some melee weapon blueprints you can buy with credits (last I checked.) Galatine is by far the most powerful melee weapon in the game, and you can buy the BP with credits. Buy the Braton if you haven't already, too.
4. Excalibur is a fine choice for a starting frame, or as a frame to just use indefinitely. Won't kill everything on the screen, but has two great oh shit buttons and Super Jump can be fun to use in combination with Heavy Impact.
5. Bosses are kind of BS in the beginning. They're targeted at geared and leveled players for the most part. They have phases and junk to stop leveled players from completely steam rolling them. Some of them are bugged too. So don't sweat not being able to solo bosses. Boss runs will always be faster and easier with more people.
6. PM your in-game name to me if you want an invite.