I usually build around the Pi pico as a brain, L9110 motor controllers, N20 DC brushed motors, and a standard 18650 lithium cell, and some generic BMS + switch mode voltage converters. From there you can either add sensors and make it autonomous (more challenging), or just control it via your smartphone (easier). You can either make it omnidirectional with mecanum wheels, (more expensive) or turn/forward/back motion only with a differential drive.
Along the way you’ll learn to solder and code, if you don’t know already. It’s a suitable beginner to intermediate project. Most of the work is knowing what cheap parts work well together (read and interpret lots of datasheets), actually assembling and using the robot is pretty easy. Usually I can keep cost under 50$, but parts are cheap here – certainly under 75$ in the West though.