The availability of dies is very much a function of their popularity. You may not be able to get dies for .223, .308 and so on, but 6mmBR are readily available, I just checked at MidwayUSA. It will cost you between $100 and $200 or more for those dies, depending on the brand and model.
As for brass, the same thing occurs there. At PVI, you can get Lapua 6mmBR for $82/100. They also have .243 Win for $92/100. The other brands appear to be backordered for .243 Winchester.
So, definitely a 6br or Dasher or something similar will be easier to obtain.
As stated earlier, the straight .243 Winchester is a barrel burner and if you plan to get into F-Open in a proper way, you need to be aware of that fact. It burns the barrel needlessly, especially for the short range. It also seems to use 30-50% more powder than the 6mmBR.
Now for some of your other questions:
Stainless Vs blued: Accuracy wise, no real difference. However it has been reported that stainless steel barrels may keep their accuracy somewhat longer, but they have the habit of losing their accuracy in a very sudden manner. I have heard of rifles giving up the ghost in the middle of a match, or from one week to the next. CM barrels slowly lose their accuracy in a more control manner. Also, you can bead blast a stainless steel barrel and that promotes cooling, whereas the blued barrels have mirror finish thus presenting the smallest surface possible and slower heat transfer.
Also, many (most?) custom barrelmakers only have stainless barrels.
For contour, that will depend on the barrelmaker and what their contours are. In F-Open, with such a high limit on rifle weight, you would be better served to have the heaviest contour you can fit. Heavy barrels are huge heat sinks, have large surfaces for faster cooling and these thick barrels are rigid. In F-TR we have to be more circumspect in choosing the barrel contour.
