no if he has a permit
You need to tell us which rifle. Chances are that there is some legal way to own it.
Depends on where you are.
Yes
Depends on the laws where you live.
Laws on this vary by country.
Yes- air weapons are amongst the few firearms it remains legal for a private individual to own in the UK.
Federal law says 18.
Yes, but there ARE restrictions on WHICH rifles, as well as which features are permissible.
Whether it is illegal to own an M90 rifle depends on where one lives. Most nations limit or prohibit such rifles. However, they are legal in most of the United States.
18 for a shotgun/hunting rifle, and 21 for a handgun.
Provided you follow all the appropriate laws, yes.
That all depends on you, and what you want. Are you looking for a plinker, a serious hunting rifle, a SHTF weapon, something collectible, what? Mosin Nagants seem to be pretty popular these days. One might consider those pretty essential.