Not all printers are in the multi-millions a year bracket, where capital expenditure in the CTP department is often less than the cost of the CEO car (not to speak of the obligatory company jet). This disclairemer having been brought upfront here are the questions:

For a small newspaper printing operation, running a couple of Goss units for exemple, doing -patiently- 4/4 or -usually- direct litho on one web an b/w on the others. Printing from 2,500 to 10,000 copies at most (with the one or twice a year exception obviously) at most 100 to 200 plates a week, needing 10 to 16 plates at a time at most (closer to 10-12 than 16). Since so few plates are done and obviously in a non-union environment, labor cost is not an issue per say, automation is then an option. Unless the equipment and workflow is so complicated that one's need a rocket scientist to get the thing running.

Killer question 1:What is the best (or recommandable) used machine available?

Killer question 2: A lot of commercial work is done on polyplate. Why is it that I have never been able to find a small web printer who uses them?

At that usage level, 100 to 200 plates a week there's no need for automation really and i don't think that automation on the computer to plate line would ROI at that low number of plates to make.

I would still get a top rated imager that's used and in good condition, without automation. Thing about automation is that probably 80% of your downtime and failures would be on the automation line and so that accounts for 1/2 of all maintennce agreement costs usually. The automation usually costs as much as the imaging unit itself too. Trendsetter Quantum and Fuji Javelin (DS Screen PlateRite) are your best bets as to reliability.

Never heard of poly plates used on a web press. AFAIK the poly use is only on the sheetfed side as far as I know. Under your usage though run lengths of 10,000 or less seems like you could run poly if you can clamp it and hold it in place. Since you cannot run poly for 4/4 then it would complicate things. Seems like metal plates on a best of class manual device with external drum and thermal quality imaging would be the best way to go for you?

At this site URL there's info toward bottom on general cost of used CTP devices. At left there are most of the used CTP vendors advertisements, they all advertise there on this site. http://Computer-to-Plate-printing-CTP.com
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A small newspaper, if they absolutely cannot afford the big 3, Agfa, Kodak, Fuji then can also look at the Screen Newsrite with Southern Litho plates.

It can be upgraded, it is available at an afforable price, and the engine is very stable stable. At the speeds the screen runs, thermal is a good choice.

I hope that helps.