definitely a long action.. there are a few who will say you can do it on a short action, but its not practical and an utter ball ache.
Ive been round this buoy with the same calibre a few years back.. As for make/model, i'd be looking at things such as availability of mounts for that action type, need for replacement triggers vs. availability of them, need for replacement stocks, and again the availability.
IME Sako 75 (action IV) is awesome in the respect its well built, great trigger and a great mag system that feeds such likes of a 6.5x284. Cons with this, limited mounting options, limited S/H custom stocks to fit.
Remington 700s pros is that you can accessorise as much as you want with ease, cons are that you need to because the trigger and some of the stocks on remmys can leave a lot to be desired.
Tikka (pre T3s) have similar pros and cons to the sakos for obvious reasons..
If money isnt a huge issue, i'd be looking at some of the custom actions, Stiller predators are easy to get hold of, are a remington clone and well made. only thing i add, which again is my own experience is that a custom action generally doesnt bring anything to the overall rifle other than looking nice and being well made. It has little affect on accuracy (with the type of cal you're looking at..).
I would be looking for a remmy 700 and have the action blueprinted if you really wanted to, or a sako75 if your not a tackle tart..
Im sure you'll choose wisely, after all you have already picked the superior calibre!