The 243 is an excellent cartridge in my opinion, but doing a custom build will give you a much better rifle for longrange. Not just for better accuracy, but because you need a faster twist than commonly available in factory 243's to stablize the high BC bullets that give you favorable longrange ballistics. It seems to be very popular among the varmint crowd to use light bullets at very hgih speeds to give the best trajectory for under 500yds (also best for gut splattering impacts). Since you are talking about going beyond 500yds the faster twist will likely be your best bet. I use a 1:8 twist but I know guys going 1:7.5 for the 115gr DTAC and others. I use the 108gr Berger BT (.546 BC) my rifle really seems to like them and I can mag feed in a short action while still touching the lands.
I know how you feel when it comes to spending dough on a custom rifle. If you want to save a few bucks you could go with a Savage and build it your self. I've built several rifle this way with excellent results. I just finished a 7wsm pictured here...
I haven't got a load worked up that it likes yet but I've only shot it once. I built this 308 and it is a 1/2 MOA out to 500yds, not increadable, but excellent for what I spent.
I'm partial to the Remington action (it's what I grew up with and just feels nicer IMO). If you feel the same, have a smith build you something you'll love for years to come.