not sure about Piston sound but usually Barring of Compressor gives sound and you have to change the barring only, no need to change whole compressor is its just the barring issue. my compressor's barring gave sound as well but it went away it self after some time. but if you trust your AC guy then i say go ahead with the change, as i can only guess sitting from here.
AC service costs around 2.5k, they will take out your condenser under the dashboard, clean it and then install it back and then fill the AC Gas. some AC guys do not take out the condenser and just wash it while it is fitted in your car. in this later procedure you wont have to install the AC gas again and its cheaper.
lastly as for Duet Compressor, Compressor price last i checked was 5.5k - 6.5k, i got it installed for 6k. then their is a bracket which costed 1.5k and i guess alteration was within the that 1.5k of bracket price, lastly they will take 2.5k for refilling gas. so it rounds upto around 10k. and no condenser will remain same.
i am not sure about terrios kid's compressor.