The world is pretty much your oyster here.
Firstly roos are not hard to kill, I think I've shot most of mine with a .22 BRNO, the most important part is to "hit" them where it hurts. I will not recommend a particular rifle, as much of it comes down to personal choice, but the first thing is that if roos are going to be your main game you do not need a heavy calibre. Look for a calibre that is popular so that you will find ammo stocked in most places, steer clear of the "wildcat calibres" and among what's left try for the flattest shooting load that you can get.
There's few things more frustrating than being somewhere out bush, and running out of ammo for your you beaut Mach 4 in .177, or whatever, knowing that there's no place that stocks that calibre ammo within two hundred miles.
Buy yourself a good brand name steel tubed scope and rings of about 6X. I dont like variables powered scopes, but that's just my personal preference.
I've never spent great amounts of money on roo shooting rifles, and usually went for military calibres as the ammo was plentiful and cheap, but that's overkill really, it's all down to what you would like. Generally you spend a lot of time walking and running around the back blocks so weight can be a consideration.
I once bought a Mannlicher Schoenauer in 243. I waited six months for it to be imported and when it arrived I absolutely loved it's craftsmanship, but really, I had just as much success and twice as much fun with my SLRs (Yeah, I know, you can't have them any more).
A good shot with a medium cost rifle will have a lot more fun than a poser with a multi thousand dollar custom rifle and scope, that he's constantly worried about scratching or getting dust into or getting wet.
Look around, ask others who do the same type of shooting, and ask if you can have a shot out of their rifles. It's like buying a car, we all have our preferences. Many of my shooting companions used to rib me about using military calibres, but I used to get as many clean kills as they did, so it didn't really count for much. I always preferred 7.62, but my son was an armourer with the SAS and he shoots almost all 5.56, I call them Kiddie calibres, but he's now a better shot than I am so I've got to keep my flap buttoned up.
What ever you do, do not buy on someone elses recommendation, if it doesn't suit you, you'll never be happy with it and as a consequence you'll never shoot it enough to really get good with it. You'll probably end up selling it for a song, just to get rid of it.