side car

The final screen resolution on the SideCar display (compatible iPad) is solely dependent upon the resolution capabilities of the iPad itself. The iPad Pro therefore is probably the best option considering the the resolution is better in these models than the other iPad models. Note that you will be hard pressed to match the resolution of your MacBook's Retina display in an iPad.

Axel F.

Sidecar system requirements

Sidecar requires a compatible Mac using macOS Catalina and a compatible iPad using iPadOS 13:

Mac using macOS Catalina

• MacBook Pro introduced in 2016 or later
• MacBook introduced in 2016 or later
• MacBook Air introduced in 2018 or later
• iMac introduced in 2017 or later, plus iMac (Retina 5K, 27-inch, Late 2015)
• iMac Pro
• Mac mini introduced in 2018 or later
• Mac Pro introduced in 2019
•   

iPad using iPadOS 13

• iPad Pro: all models
• iPad (6th generation) or later
• iPad mini (5th generation)
• iPad Air (3rd generation)

Additional requirements

• Both devices must be signed in to iCloud with the same Apple ID using two-factor authentication.
• To use Sidecar wirelessly, both devices must be within 10 meters (30 feet) of each other and have Bluetooth, Wi-Fi, and Handoff turned on. Also make sure that the iPad is not sharing its cellular connection and the Mac is not sharing its Internet connection.
• To use Sidecar over USB, make sure that your iPad is set to trust your Mac.

Axel F.

Great idea. This way there is no doubt in your mind.

Just FYI - It may be a lot cheaper to buy an external monitor/display to work with your 2018 MacBook Air than buying an iPad Pro just for this purpose. Of course, if you were going to buy an iPad Pro for other reasons regardless then your thought process makes sense. Good Luck.

Axel F.