First consider how fast of a copier you need. If you can get away with four or five pages a minute you will end up paying a lot less. On the other hand if you are preparing small booklets to send out to your customers you may want a machine with more horsepower and additional features like a sheet feeder and a staple finisher.
Do you use different sizes of paper very often? If so a copier with a couple of drawers with different sizes of paper can be really handy. Nothing is more aggravating than copying something onto the wrong size paper because you or someone else put bigger paper in for the last batch.
If you are patient and organized, copying a few sheets of paper on both sides isn’t much of a problem. You just copy one side and take the copies out and put them upside-down into the feed tray and copy the second side. However it is easy to copy page 3 onto the backside of page 2 if you aren’t careful. Copiers can turn the page over for you and copy onto the backside if it has a duplex feature.
Take a good look at what you actually do and think ahead about what features will come in handy.