hey jord!
One of my fellow music major bass buddies at school uses that EXACT rig (avatar 210 with GK backline 600) and he loves his. I have heard it and it sounds great! I have no doubt in my mind that setup would sound great.
As far as being able to add on more in the future, the 210 cab will prove to be perfect. the avatar 210 cab in 8 ohms will allow you to add any other 8 ohm cab to the rig later! and the 210 is also a great cab by itself.
the head will still work if you add the second cab later, but in the future if you want to move some SERIOUS air, you MIGHT find the 300 watts to not have the headroom you need to run 2 cabinets very loudly. This is typical of a starting rig though, the head might end up being something you want to upgrade in the future (maybe something around 500-700 watts depending on your situation) but the avatar 210 cab definitly won't need upgrading, its a great high quality cab and you can get another one if you need more volume later. you might find the 300 watts is plenty for you to run 2 cabs with (it CERTAINLY is enough for 1 cab)
you have been saving for a long time, and you will love the rig when you get it. its easy to worry about having to upgrade in the future, but in reality, I don't know anybody who hasn't gone through several amp rigs. there will always be something bigger, louder, and more appealing. If you really do find yourself in a situation later where you truly need something bigger and you have to consider switching to a more powerful head, just consider it a sign that youre advancing in your musical career. in the mean time, I really think you will be happy with the avatar/gk setup.
I have used avatar in the past, and still use GK right now, and I love both. I strongly consider both of these brands to offer the best bang for the buck in cabs and heads, respectively.