I hope this is not a dumb question. I am going to replace both my chain and derailleur. The old chain was fine, even though it was designed for a seven speed rear sprocket with a 46 tooth front sprocket. The bike now has a five speed rear and a 52/40 front. Now the chain is in need of replacement. My derailleur also is old; the small gears are getting round. The derailleur itself is pretty banged up too.

I am asking if anyone has an opinion on what I should replace first, the derailleur or the chain.

I also have a question on the chain length. I watched on line videos on how to measure the length of the chain for installation and it seems simple. I was wondering why I could not just use the old chain as a gauge.
Any opinions would be welcome, TM.