G'day Shooter243,
I think most people have hit the nail on the head here as far as getting a 22LR to start practicing with.
If the maximun range that you can generally get to 200m with you having to travel to get out to a 600m range, my advice is this-
Grab yourself a HOWA 1500 Barreled action or one of the cheaper rifles(1 in 9 twist). same same for the your local store.
Dont stress about the stock because you are going to want to drop it into a Bell and Carlson or one of the many chassis that are going around. up to you if you want a detachable magazine or not. last but not least is glass, i highly reconmend bushnell, recently got a one of the NITRO line and by gods am i impressed with it. its no nightforce or schmidt but it definately can compete with them as far as quality for price goes.
Im saying this because i turned my Weatherby Vanguard (fancy HOWA) into a tack driver by doing just that.
The B&C stocks are alloy V-block bedded to get the best action to stock fit you can short of glass bedding the whole rifle.
The big thing is practicing, you can train all day on a rimfire but you will still need centre fire time. by going with a 223 you can buy those OSA bulk ammo deals and shoot to your hearts content. that little 223 with 55/60grain projectiles is more than capable of half minute groups at 200-300 from a well setup factory rifle.
All in just my 2cents, i wish you the best of luck with which ever direction you choose to go and am looking to forward to hearing your results, better yet seeing you on the range mate.