As the manufacture of the military load 8x57IS was prohibited after WW1, the old "Versailles Treaty Cartridge" 8x60 was developed as a pure sporting cartridge to replace it. It's design allowed existing 8x57 rifles to be rechambered. It has a longer body than the 8x57, 48.2 vs. 46.2mm, so the use of 8x57IS cartridges can not be recommended! The 8x64 has a body length of 51.8mm, so firing the both shorter cartridges in that chamber would be dangerous because of grossly excessive headspace.
Making 8x60 brass from 30-06 is easy: Open the neck to 8mm and run into a full length 8x60 fl sizing die, trim to 60mm. When out of original RWS cases, I use Sellier & Bellot 30-06 brass myself. RWS 30-06 brass is less useable, because it gets thicker at the neck/shoulder junction. Setting the shoulder back on RWS cases leves a "donut" on the new neck, requiring neck reaming or turning. I have not yet tried Lapua or Norma brass. IMHO starting with 7x64 or 9.3x62 brass would only complicate things.
8x57IS load data may be safely used in the 8x60S. In fact, it may even be increased slightly by about 3%. My own favourite loads for the 8x60S, RWS brass:
1. 200 grain .323" Speer pointed soft point in front of 50 grains VV N140 for 2600 fps i.v. from my Mannlicher-Schoenauer 23" barrel. Shot all sorts of European game with this load.
2. If the rifling twist used by Sako stabilizes it, 250gr Woodleigh rn .323", 53 gr VV N160 for 2330 fps. This load is fully equal to the fabled .318 Westley-Richards, but with markedly less problems of getting proper diameter bullets.
Rechambering to 8x64S is not practical, as the 8x60S has a larger chamber neck diameter, 9.1 vs 8.99mm. The new chamber will produce a marked step halfway up the neck on the fired cases. A rechamber job to 8x68S will require more gunsmithing than you imagine:Though the magazine is long enough to accept the 8x68S factory loads, reliable and smooth feeding of the fatter cartridge requires extensive modifications of magazine follower, feed rails and receiver ring chute, done by a man who thoroughly knows what he is doing. So I would leave the 8x60 chambering as it is, as you will gain little from rechambering.
A final warning: Many early post-WW2 Sako rifles in 8x60 are barreled for the 8x60I, .318" bullets instead of the 8x60S, .323". So inspect the rifle carefully before purchasing.