I need help to decide which of the Arduino board is the ideal to manufacture a RFID reader that works standalone for indefinite period.
I suggest you start by identifying your requirements i.e. in terms of the microcontroller speed, memory, flash etc, the number of pins of each type, and the voltage/current constraints of your power supply, and any ancillary devices you need to provide.
Is building your own out of the question? Working on such a system now to provide access control to stuff in a makerspace. 
I'll probably end up using an Uno for testing, then move that to a standalone 328-based PCB with all the reader module goodies. Too bulky and fragile to use the larger boards, although the Minis and eBay knockoffs are easier to integrate I guess. I'm just too DIY for that. heheh
although the Minis and eBay knockoffs are easier to integrate I guess.
Integrate is a broad word... can imply physical aspects that are difficult to match with pre-manufactured boards. However, easier to get from the receiving box to the project prototype, Yes.
In small quantities, I cannot produce this at the low-price of under $6.40 delivered! This particular Hong Kong seller has numerious RFID cards and tokens.
Ray
