Porting my project to multiple Nanos
The Arduino Uno and Arduino Nano are fairly compatible.

Problems:


The board layout isn't the same, Nano's won't wield a classic "shield".
Nano's don't have a barrel jack.
Nano's (usually) have a micro USB connector. While the Uno has another one.
But, Nano's are smaller, withouth leaving out the USB connection.


But wait!

Microcontrollers of the same family are quite scaleable, Nano and Uno might even have the same micro on it! Be sure your Nano has the same Atmega328(P?) chip as your Uno.
Check if the pins you're using, are accessible on the Nano. Then you should be fine.

There is a little problem with cheap Arduino Nano's, I've bought 2, from 2 different vendors. They we're about 3$.


The first one featured a counterfeit FTDI chip. By installing the latest FTDI drivers, my FTDI actually got bricked. I had to undo this by using M_PROG or some EEPROM flashing tool to unbrick the chip, and install older drivers (which didn't corrupt it).
The second one featured an CH340G usb-serial chip. I had to download and install the drivers from a chinese website. But Windows wouldn't install unsinged drivers that easily. You have to go into advanced boot options mode and allow installing unsigned drivers.


Getting these Nano's working took me an reasonable amount of time. But they now are working quite well :)
It's something to take into account when buying cheap.

I wouldn't really bother for the alternative crystal, as long as they're 16Mhz. For time-critical behaviour, I wouldn't rely too much on your crystal.