What should I upgrade first?

To start with there really is no WiFi or Bluetooth upgrade for your system: 802.11a/b/g/n & Bluetooth 2.1 + EDR as the interface is different for the newer systems. There was a point a few companies offered upgrade kits but that ship has sailed with an adapter and needed cables to convert things.

The newer 2013 27” offered 802.11ac but this is the Phase-1 version & Bluetooth 4.0. It wasn’t until 2017 when Apple offered 802.11ac Phase-2 and Bluetooth 4.2 and it wasn’t until 2020 when they offered Bluetooth 5.0.

Frankly its cheaper to just use Ethernet 10/100/1000BASE-T (RJ-45) connection either directly to your internet router or use an Ethernet to WiFi bridge to connect to your main routers AP if you don’t have a direct wire connection path.

Your systems drive connections are limited to SATA III (6.0 Gb/s) drives. But jumping to even a 2.5” SATA SSD drive will get you much more performance either adding it as a second drive or swapping out your 3.5” HDD drive.

• iMac Intel 27" EMC 2429 Austausch der Festplatte
• iMac Intel 27" EMC 2429 zweites Laufwerk (SSD oder HDD) installieren

So the order I would do is first upgrade the RAM as that’s the easiest. Second I would upgrade the HDD or add in a SSD drive. Jumping to a i7 CPU is the most work and depending on the app may not get you that much performance unlike the RAM & Drive upgrades.

Now the next thing is the consider how much this will cost and if you can find even a used newer system which already has what you want for CPU, RAM and drive.

I would spend some time looking around a bit as you might be surprised what you can find. I helped a friend find a nice 2014 27” iMac for $500 which we added RAM and a PCIe/NVMe SSD pushing it to the max for $450 so it ended up cheaper than even what it was sold as new.