Jeepkid-a company called canjar in denver co has been making excellent quality single set triggers for some years and I wholeheartedly reccomend them. They are available for most centerfire major brand rifles. I have tried just about every trigger available when building rifles and have yet to find a trigger that will even come close to compare to-let alone best canjar triggers. They are reasonablely priced ($150-200),and require very little fitting/adjustment,FULLY adjustable,and simple to install.
As to cartridge,I really don't think there's a lot of difference in the different wildcats unless you choose a exotic wildcat that's going to burn barrels up quickly. Another thing to consider is as velocity goes way up,you meet the point of diminishing returns on amount of powder vs velocity gain. Also,bullet selection becomes more critical for good terminal performance.