IMO, just starting out, try something tested and true and with lots of components on the market to make things more available for your testing.Lots of this stuff available here on EE forums(.308 stuff , .260 , etc...) A lot of those standard calibers that guys are suggesting are perfectly fine for 700m. I have to agree with staying away from the barrel burners, especially if on a budget.
Big case capacity is usually followed by big recoil which will effect your accuracy if doing any amount of shooting.Speed has nothing to do with accuracy, it is whatever load your rifles winds up liking that will give you your accuracy.
If someone told me to pick just one, most likely .308. Easy to get barrels also. Talk to Obtunded (Ian)about custom barrels, he may even have one ready to go for ya!