Hacksaw is the cheapest but the hardest work (not good when on your back underneath a car).
Grinder is possibly less controllable for a square cut.
Chain type cutter (your post 3) - does it have to be rotated through 360 degrees around the pipe? (might not have the clearance since the old one is on the car).
Reciprocating saw?
Sealey VS16371 - too expensive.