monkey44, your not going to "make a mistake" if you purchase through your current cell phone company.
The phone should work fine and should have similar operating range.
There may be a couple of other brands available that are not offered by your cell phone company but in reality a flip phone is a flip phone.
It is there to mainly make and receive phone calls and texts, well the phone keypad dance is one of the reasons I found a smart phone to be nicer to use.
Something to consider though, unless they have added call and text blocking like smart phones you might be wishing you had that feature. Especially since you are adding an extension which results in getting a new number for that phone. Phone companies reuse a lot of numbers when old customers drop service so it is often the luck of the draw and you might get a number that spammers and scammers abused a lot.
Something else to consider, there is newer smart phones out that fold up like a flip phone.. They use a new folding glass technology LCD which bends. Samsung Galaxy Z Flip is one example that I can think of right now that may be a good choice. Not sure of cost and longevity of the bending glass but gets you a flip phone that folds compact, has soft keypad for texting plus should have call and text blocking for the annoying spammers.. Kind of the best of flip and smart phones.