I do not think an all-rounder worthwhile owning for a lifetime of reliable use and enjoyment can also be cheap. And in the long run you will be happier with a more solidly built, durable, shotgun that has many after-market barrel and accessory options.
It sounds that you want a good do-everything 12 ga shotgun, and you want it for life - which is great - everyone should have such a shotgun. (Another reason for spending a little more up front for long term performance and enjoyment of the shotgun). I have such a do-everything shotgun: an older model Remington 870 Wingmaster, with waterfowl barrel and short smooth bore "deer" barrel with iron sights and with a variety of chokes, one of which is rifled for spinning a sabot slug. And you can get fully rifled slug barrels for the 870 if you want. It has a plethora of after market barrels and parts for everything you could imagine.
If you want reliability when a bear is charging, I would recommend NOT going cheap. And since you want interchangeable barrels and choke systems, this is another reason not to go super cheap. You want long term availability of parts and upgrades.
In what ever shotgun make and model you chose, I recommend a pump action for the reliable do-everything system. Train with it and the pump action operation will become muscle memory and reflex. Practicing with two friends flinging 2 clay birds at a time is great training for pumping fast to get that 2nd clay bird before it hits the ground. Since you are a beginner in shotgun shooting, you will find that occasionally you will need 2 rounds to smoke just one clay bird (maybe 3)
If you are going to competently dispatch a predatory bear, it helps to be proficient in your shotgun handling and have confidence in your skills. This means practice time, and it means wear on the shotgun - another reason to buy quality.
Best wishes for your choice.