Because you mentioned Moose to 400 yards, 1200 yard steel, and factory ammo, that to me, as others have already stated, says 300 WM is the minimum. It can be done with a 308 or 30/06 but the 300 allows for ammo to be loaded up and down accordingly. He can handle a little recoil but once you go up from the 300 WM it goes up considerably. I think the 308 is a great caliber to learn on and become proficient with and more than adequate for bear country but if Moose are on the table beyond a few hundred yards it is insufficient imho.
As for those mentioning the 6.5 and 7mm, I love both calibers and for 1200 yard steal just about anything in those calibers will do the job. Europeans have been killing everything on the planet with them with them for 100 years. I have personally witnessed two Moose (Norwegian elk, which are smaller than their North American counter part) killed with the 6.5x55, but both shots were inside 100 yards and shots on larger game are usually head shots. If Moose are on the table beyond a few hundred yards, which is a discussion for another thread, the 300 WM is just a better choice and about perfect for your requirements. I personally do 90% of my hunting with a 270 win. but when in big bear country I opt for at least a 30 cal option. Caliber: 300 WM It's essentially an AI 300 H&H and its been getting the job done for over 50 years, use enough gun.
As for the gun you should be able to find plenty of used options in a Tikka, Sako A7 or TRG-S or Weatherby Mark V Fibermark or synthetic for around the $750 mark or less. You might even find a new Tikka TX3 which has a huge amount of aftermarket support.
Scopes: Look for a used VX3 or Mark 4 3.5-10x40/50 with a mil dot. $499 est. and up…
New Scopes: These are recommended because you are looking at shooting 1200 yard steel. Listed most expensive to least and best for application to good.
Leupold VX5HD 3-15x44 CDS HTMR $1099.99
Athlon Midas Tac 5-25x56 APRS6 $850.00
Leupold VX-R 3-9x40 or 4-12 TMR w/ Firedot $700
Leupold VX Freedom TMR $499
Athlon Argos BTR Gen 2 $350.00
(All of these maybe less if you find them on sale.)
Rings: Hawkins Percision Ultra Light Tac with bubble level $120-160
This is a Sako A7 Roughtech Range in 300 WM with an Athlon Midas Tac 5-25 and the Atlas works upgraded bolt handle and bolt shroud. Upgraded bottom Metal on the way $169. It has an APA LB gen2 break on it. Literally, shoots like a 223. Gun $1100, Scope $799, Rings $120, Atlasworks upgraded $120, break $120 Bipod Harris $100. Rear pad $30. Shoots sub 1/2 moa. You should be able to find a used gun or even a discounted new one for around the $850 mark.