1) Only heards that are not in certain areas and thats an artifact from cattle ranches. In most of the lowveld the buff are long-standing populations and certainly not quasi-domestic! However it is true that areas with substantional HUNTABLE populations of buff are quite limited (the classic lowveld areas).
2) I think NORMALLY a 600yrd shot is not acceptable but I feel in this case with a client who clearly shoots at those ranges and has the appropriate gear, its a different story, still more like 'sniping' BUT each to his own and clearly he can pull it off competantly. The ethics of this become very blurred, not well defined.
Allow me to relate a similar story form an old PH and ex-boss of mine (Ian Goss). He had a similar client who was a very good long range shooter and was using some super-duper ultra magnum tactical type rig. They too spotted an Eland at 500+ yards and the client said he could take it from there. Ian said NO WAY; waaaayyy to far. The client then asked Ian if he was using a .30-06 and that was a Kudu at 250yrds would he let him shoot..Ian said yes and then realized the guy had essentially asked to extrapolate the proportionate distances, animal sizes and trajectories....needless to say he then aggreed, somewhat relactantly, and the eland was dropped.
I would say that 600yrds is totally out of the question 99% of the time...but I would first learn about the shooter and context before jumping down his throat. VERY few guys have the right gear and more importantly the right skills, but some guys really do. Not my cup of tea but whenever I see the long range guys at work (only a couple of times) its very impressive. We can argue ad nauseum about hunting vs. sniping but in this case the guy appears to be competant in this regard.
3) I have never shot a buff but also know a few guys who have and don't know anyone who used a solid as a first shot for the last 20years. Premium soft first, solids underneath.
Kudu - kalmeer boet, ons almal hou van skiet en jag and party van hierdie ouens is regatig baie ervare, afstaande van waar hulle jag. Cheers.
4) The OP - take both, either will work but a spare is nice to have. If your outfitter does have a loaner rifle that is decent and guarantees it will be available then take just one. However you wouldn't want to have it do down and then learn that the company rilfe is being used elsewhere....