Build a .223 rifle with a TL-3 action, KRG Bravo chassis, trigger of your choice, ARC M10 rings and a good optic.
When you out grow the .223, get a 6mm/6.5mm barrel of your choice and a bolt head and get after it. You can do a lot with a .223, including 1,000+ yard shooting.
Alternatively, you can build something similar to the above but in a 6BR/6BRA. Low recoil, Lazer beam accurate so it's fun to shoot at 100 yards, can easily go 1,000+ yards for a training class. I think a 6BR would be a great all-around cartridge.
What I did for my wife, was build her a 6BRA rifle. I wanted to build her a good all-around rifle that had low recoil, great accuracy, and was a very capable rifle at most any distance (short of ELR). It ended up being a Lone Peak Fuzion action, KRG Bravo chassis, BnA Tacsport trigger, ARC M10 rings and a Steiner M5 scope (which I had laying around).
More rifles is always nice, but I think it would be best to start with one rifle and work with that until you start to learn your preferences for triggers, chassis/stocks, optics and reticles, cartridges, etc. It gets expensive if you have to replace or upgrade two rifles as your needs and wants change versus one. I think you can accomplish everything you want with the one rifle.