Everything on a bicycle is replaceable except the frame
The frame
Look for dents behind the head tube and rust from the inside at the bottom bracket. Both are fatal and neither is repairable economically.
The wheels
Spin each and watch the gap at the brake. More than three millimetres of wobble means a rebuild, which is sixty pounds a wheel.
Everything else
Chain, cassette, brake pads, cables, and tyres are all consumables. Assume you will replace all of them and price accordingly.
