Is It Worth Refurbishing My MacBook Pro (Retina, 15-inch, Mid 2014)

If you are not looking for any hardware upgrades, then the only things you can do are to upgrade macOS if it hasn't been upgraded already or to perform a clean install of macOS (erasing the drive before re-installing macOS) and manually migrating your data and manually reinstalling your apps. The upgrades are free so it doesn't cost you anything more than the time it takes to upgrade the OS and possibly your apps. Performing a clean install is just a time consuming process depending on how many third party apps you have and how much you customize things from their default settings, but it can be a good thing to do sometimes especially if you've upgraded the OS multiple times or installed/removed lots of third party apps.

Your laptop can run the current version of macOS which is 10.15 Catalina. Keep in mind that Catalina no longer supports 32 bit apps so make sure to either upgrade any 32 bit apps first or uninstall those 32 bit apps before upgrading to Catalina. Here is some information about 32 bit apps:

https://support.apple.com/en-us/HT208436

If you need to run 32 bit apps, then you will only want to upgrade macOS to 10.14 Mojave.

Regardless, you should always make sure that all your third party apps are compatible with the version of macOS you will be upgrading to (whether 32 bit or 64 bit) since some developers may have stopped supporting some apps with later versions of macOS.

You can find links to versions of macOS from 10.11 through 10.15 here:

https://support.apple.com/en-us/HT201372

I highly recommend you have a good backup before attempting an OS upgrade. FYI, you should always have frequent & regular backups in case of any emergency or accident. I also recommend creating the bootable macOS USB installer as well as it gives you more options if something goes wrong with the OS upgrade.

The Retina laptops use a proprietary Apple PCIe SSD. OWC makes an SSD which fits this proprietary connector.

https://eshop.macsales.com/shop/ssd/owc

You will need to already have macOS 10.13+ installed before you can install a third party SSD in this laptop since the laptop needs a system firmware only available with macOS 10.13+ installers. You also can only use macOS 10.13+ when using a third party SSD in this laptop.

I highly recommend you keep the original Apple SSD just in case it is ever needed to perform a system firmware update or to upgrade macOS in the future.

Edit: The memory is not upgradable since it is soldered to the Logic Board. You can upgrade macOS to 10.15 Catalina (as long as you don't need any 32 bit apps).