The best professional camera to buy under a small budget is the Canon EOS R100. It is a compact mirrorless camera with excellent features for both photos and videos. It has a 24.2-megapixel APS-C sensor, Dual Pixel AF for fast and accurate autofocus, and can shoot 4K video. It is also very affordable, priced under $1000. Additionally, it has a vari-angle touchscreen and built-in Wi-Fi and Bluetooth for easy sharing and remote control. It's a great option for those who want a high-quality camera that won't break the bank.