I've always held to the philosophy that one "needs" 5 guns; everything else comes under the heading of "want".
All you "need" to do pretty much any reasonable task with a firearm are rimfire rifle, rimfire handgun, centerfire rifle, centerfire handgun, and a shotgun.
Since those are the basic needs, they will see a lot of use. It follows that ammunition, magazines, parts, etc. should be both cheap and readily available.
Since you asked, I would add a .22LR bolt rifle (another Savage) and a 12 gauge pump shotgun with 26 or 28 inch choke-tubed barrel and 18 inch rifle-sighted barrel (Rem 870 or Mossberg 500 combo). Handguns, too, if you want.
Alternatively, since you are used to single-shots, a Rossi or H&R rimfire/shotgun switch-barrel combo would be cost-effective.
H&R / Marlin (now owned by Remington) may still be doing their "add a barrel" program, where you send your SB2 receiver in to be fitted with as many spare barrels as you can afford. Last I checked, they were about $90 each.