Diamond team review: could use help with a team build

I currently have two gym badges. And I could use some advice on my line up and strategy for going further!
I'm new to the modern generations so I have no idea of what Pokemon I need to keep my eyes open for. I know I have some gaps in my offense and defense so I would love some advice! Thank you!
My current line up is...
Lvl: 18 buizel
Lvl: 21 luxio
Lvl: 21 monferno
Lvl: 21 staravia
Lvl: 18 kadabra
And an egg from "traveling man"?

Actually your team isn't that bad to be completely honest. You have access to just about every offensive type you could hope for at the moment so you'll have something for every situation. You don't really have a defensive Pokemon, however, that egg should remedy that if you want to have one on your team. You just might not get a lot out of it for tanking physical hits. That aside you could easily fit a steel type into your team somewhere, there's quite a few in the region. Bronzor evolves into a fairly capable Steel/Psychic type that's capable of taking hits. You can also pick up a Nosepass to let it evolve into Probopass to give you a defensive Pokemon with the ability to use Thunderwave to cripple faster threats for your team. On top of that all since you're playing Diamond you can pick up an Aron in the future as well. You have quite a few choices for your team for a 'defensive' Pokemon if you desire one. Nosepass and Bronzor can be found in Mt. Cornet and areas around it, and Aron can be found at Fuego Ironworks.

One other suggestion I have is if you don't have anyone to trade with, you should try and hunt down a Ralts on Route 209. It's similar to the Abra line except you don't need to trade to reach its final form. Have fun playing the game!

Ralts is only a Platinum thing.