Sous vide cooking is often at low temperatures, for long periods of time. But, you lose out on things like evaporation which can be essential for slow cooked meals or certain types of flavor development. You can't do things like stir food in a sous vide setup easily. So you have to be careful with what recipes you convert to sous vide; some will be easier and better if you don't use sous vide.
People do use mason jars for sous vide cooking, e.g. to make cheesecakes/custards. But most things are done in plastic bags (ziploc freezer or vacuum bags).
If you'd like a setup that can sous vide and slow cook and you have an oven, you could buy a dutch oven and a sous vide circulator to put in it (e.g. something like Joule since it sticks to the bottom of the dish magnetically). For slow cooking, put the stuff in the dutch oven, leave the dutch oven covered but open by a crack and stick it in a low oven (225 F ish). For sous vide, put water in the dutch oven, put the circulator in and go. As The Food Lab states in their pressure cookers > slow cookers article, the dutch oven setup described prior will often give you better flavor development and is not necessarily any worse than slow cookers on the safety front. You also have versatility, e.g. you can increase the temperature to 300 F make a slow cooked tomato sauce and take advantage of the maillard reaction better and things like that.
Alternatively, just buy a slow cooker if you really want a slow cooker -- they're super cheap, particularly if you look at yard sales (likely under 10 dollars). The price will be basically negligible next to your sous vide circulator. Then buy whatever circulator you want and use it in an appropriate container.