Ive done three of them. Do big deal. It does take te right tools and a small amount of above average DIY skill. The mechanics story sounds fishey to me. Normally when the rails break the chain jumps and you have bent valves. If they are broken and it didnt jump timing, then there are some loose pieces running around inside your engine. All of he chains I have done used clips so no need for the crimping tool.
He claimed that the links were jumping when the car was idling upon ignition and that as the rpms rose, the "roughness" would disappear because this could not be detected at higher rpms
Sounds bogis to me. If links were jumping around, you would have bent valves.
While I was in there I replaced the valve stem seals and my oil consumption went to zero on both of my 420's. Total cost about $150 and about 6 hours time.