Re: Caliber decision.
The 7mag is a good LR round. Get a good base, rings and glass and try her out then decide.
Not sure on the resale, I would not invest in a rig with that being a factor, build what you want. If it were me building a LR rig from the ground up, I'd have to go with a 6mmBR or .264 for a AR15 or .260 in a bolt gun or AR10.