If you already have headphones/earbuds, all you need is a good quality USB microphone. The overall quality will be superior to an inexpensive microphone headset and the hardware can be multipurpose.
Microphones from
Blue are available at various prices and all of them are quite good for the money. Even the least expensive models would be suitable for your needs. They are also available on sale; I got an original Blue Yeti for $80 brand new. Just make sure that the mike is suitable for picking up your voice while sitting normally at your computer. For example, the Yeti has various options and the setting I use lets me speak at normal volume while typing on my keyboard.
If your iMac has a standard audio-out port and you already have speakers using it, an inexpensive inline "splitter" that adds a 3.5mm port is all you need. The speakers need to be powered-off or muted because the splitter adapter does not block audio from them.