? about the 358 hoosier and the 450 bushmaster....

As I understand it...The Hoosier is a custom cartridge so you need a donor bolt rifle for the barrel or you can buy a barrel for the Encore too. You will have to buy custom ammo for it if you do not reload yourself.
The 450 is a standard cartridge so you can buy production ammo and rifles.

Personally if I am using something besides a shotgun slug or muzzle loader in Indiana I would choose a Ruger Model 77/44. Currently I don't see any rifles or factory ammo being produced for the .358 Hoosier, but do see some manufacturing for the .450. I really doubt if the .358 Hoosier cartridge will become a commercial cartridge or there will be a commercial rifle chambered for it. Just like a number of cartridges, they will remain "wildcat cartridges". Generally those state that allow the use of H.P. rifles like Kentucky, Folks in these states usually choose the .243, .270, .308, or a .30-06 for their Deer hunting. If you choose a .358 Hoosier or one of the .358 WSSM you'll have to make your own ammo and have a rifle built for it.

The 450 BM compares more with the 460 S & W than with the 358 Hoosier. The 358 will either have to be reloaded for or just by your ammo from (a custom reloader such as) AJ Brown. In time, there will be other sources no doubt, even if it's never available from the factory. Currently, a box of 450 BM will run you about $20 less than custom loaded ammo available from Brown's, but the custom ammo offers much more flexibility than the choices available from the factory loads.

As for building a rifle, you really don't build it. You have someone build it for you. So, all you have to do is pay for it. The simplist way is with the Encore and just buying a barrel. Cheaper that away too than a custom rifle or the Bushmaster AR.

Not sure about different brand uppers being interchangable?

I have a 460 S&W that I hunt with and a buddy of mine uses one. Both are Encore factory barrels. The only thing is the factory Hornaday amo is tuft on deer if you hit bone. It busted up shoulder pretty bad on 2 of his deer and 1 of mine. I am hand loading 250 grain barnes Muzzle loader bullets for next year. Recoil is about the same or less than a 20 ga. both 460's shot 1' groups at a hundred yards. It's an easy 250 yard load.

If you can get a custom barrel at 24" to help with muzzle jump. I know some will number the hell out of the setup with specs, but it a great deer gun. It's a pop and flop gun.