We found used Ersa HotFlow 2/14 reflow oven that does not have bottom side preheaters. Can it be easily and safely used in normal high mix/low volume production or is it built for specific products which does not need bottom preheaters?
That model comes standard without bottom preheaters. Bottom preheaters are an option so I would think it is designed for general use unless you have some very thermally challenging boards. As long as you are within the process window, you'll be fine.
I started out with an old four zone oven with NO bottom heaters at all and we got the job done (SnPb).