The 6.5x57mm has been a popular cartridge in Europe for many, many years,,,,,,, since like shortly after 7x57mm was intoduced! RWS still loads ammo and has brass.
The 6.5x57 was a very popular round, and the rimmed version is common in break actions and drillings.
Although the 6.5X57 was a very popular round in continental Europe, it has been steadily losing ground for many years and precious few rifle manufacturers offer it these days. When everything was long action and 8X57 and 7X57 were common cartridges, it was a logical choice for a good sporting rifle built on an M93/95 or M1898 action. It also made sense in the many European countries including France, Spain and Italy that either didn't allow civilian use of rifles chambered for military cartridges at all, or if they did required much more onerous licensing and registration. However, those rules have nearly all gone in recent years.
It's fairly rare in the UK which has very much followed US trends since WW2 in both rifles (after BSA and Parker-Hale stopped trading) and even more so cartridges. The Scandi nations have always had that excellent 6.5X55 Norwegian / Swedish design, and the Finns long stuck with their native 7.62X53R and wildcats based on it before following the herd with other 6.5s, post-WW2 US magnums and because of their large moose population 9.3s in recent times.
With the little roe deer (70lb undressed max weight) the most common deer species across Continental Europe and the British Isles, a modest power 6.5mm cartridge makes a lot of sense for many hunters, but there is hardly a shortage of choice these days in this category from the 6.5mm Grendel upwards for anyone buying a practical modern stalking rifle for use in sopping wet deep forests (ie plastic stocked not beautiful custom made walnut). Same for Scandinavian reindeer and Europe's many goat and mountain sheep species. Red deer can run really big, heavy and tough across much of Continental Europe (much bigger than we see in the UK) and .30-06, 7mm Rem Mag and 300 Win Mag became increasingly popular in many countries from the 70s and 80 onward at the expense of native cartridge designs. Likewise with Wild Boar whose populations have grown to pest proportions across most of Europe in recent decades.
Assuming Frenchmen, Belgians, Italians and others disobeyed their German occupiers post 1940 and hid their hunting rifles (risking summary execution), and likewise Germans and Austrians disobeyed their Soviet or Allied military occupation government ordinances on handing in firearms post '45, I'm sure many fine 6.5X57s survived as such sporting rifles were (still are) highly regarded as family heirlooms and are passed down through the generations. Whether they remain their owners' choices for day to day hunting use may be a different matter and they're brought out instead for local and regional hunters' Schutzenfest competitions and similar outings.
So far as building a modern custom rifle goes, yes a nice project indeed and a good talking point with one's friends on the range. But as a practical proposition, if I were looking for a long-action 6.5mm project I'd go for one of the 6.5X55 improveds, like the aforementioned BJAI instead.
Lately x57 brass of any sort is not real easy to find. If that doesn't scare you & it's what you want, go for it. Huntington's has been known to stock RWS brass & ammo.
RWS is part of RUAG which is western Europe's largest supplier of military ammunition ........... and there is a war going on in Europe that grows by the week and might last a long time. I'd not bank on any RWS case being available in the short to medium term, especially the more esoteric designs.