iMac12,2

Hello wonderful folks. I was gifted a 27” iMac12,2 that is still working nicely. I got it from an office that needed to let go of it because of out of date hardware requirements for their internal systems. My dilemma with this thing is that it runs great and I even got it to update to High Sierra however I can not download any aps like numbers or even be able to sync my message in it bc the apps require a newer version in OS. I started reading into the FAQ and supported models in OCLP and it seems like there is life that can be given to this machine to allow me to use it. I do admit I am struggling to get started and understanding what I would need so hoping you all can help me with these two question.

1. How would I know what is the best version I can run in this machine?

2. What are the costs associate to such an upgrade? Considering a new mini is 599 and a new imac is 1299 of course.

Here are the tech specs

Model Name: iMac

Model Identifier: iMac12,2

Processor Name: Intel Core i5

Processor Speed: 2.7 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 6 MB

Memory: 4 GB, only 2 of the 4 slots used (accepts 1333 MHz DD3)

Graphics: AMD Radeon HD 6770M 512 MB

Mac HD 1 TB SATA disk

Bootcamp 928.34 GB, 850.72 GB avail

Bluetooth: 2.1 Broadcom USB 2046

Boot ROM Version: 87.0.0.0.0

SMC Version (system): 1.72f2

Currently running High Sierra 10.13.6

I do have two memory module MC727G/A but not sure if those fit in this