Will all L style and T style 85 watt MagSafe Power Connecters work on a Mid 2012 MacBook Pro 15" core i7 2.3 model A1286? I keep finding ones for sale that say before mid 2012, and then others that say after mid 2012, so do I get the ones before?

No, they are different… check this page out to find which model is for your Mac :

https://support.apple.com/en-us/HT201700

As long as the MagSafe connector is the same version 1 or 2 they are interchangeable (Magsafe 2 is a wider & narrower connector used on the Retina models). I personally prefer the "T" style since they tend to fit a bit better than the newer "L" style which has barely any clearance between the connector and the side of the laptop If the laptop corner by the Magsafe port is even slightly deformed it can prevent the "L" style connector from making proper contact. Unfortunately Apple only sells the newer "L" style now for the Magsafe 1 chargers.

I suggest you only buy a new charger directly from Apple or an Authorized Apple reseller since there are many counterfeit chargers online. The counterfeit chargers don't work as well and many of them are very dangerous since they are cheaply made without all the proper safety circuitry which could hurt you and damage your laptop. Apple still sells the Magsafe 1 chargers online:

https://www.apple.com/shop/product/MC556LL/B/apple-85w-magsafe-power-adapter-for-15-and-17-inch-macbook-pro