When sourcing cheap mountain bikes, focus on the frame material, typically 6061 Aluminum Alloy for a balance of weight and durability, or High-Tensile Steel for maximum cost reduction. Ensure the drivetrain components (derailleurs and shifters) are from reputable brands like Shimano or L-TWOO to guarantee shifting reliability. For safety, prioritize mechanical disc brakes over V-brakes, as they provide superior stopping power in wet conditions. Additionally, verify that the suspension fork has at least 80mm-100mm of travel to handle basic off-road terrain effectively.