There are some off-the-shelf Instrumentation Amplifiers that would simplify your input circuit and would work rail-to-rail making it well suited to using the built-in A/D input of the arduino. Analog Devices has specialized in medical grade devices for many years. At about $7.00 the AD8220 is kind of pricey but it would be a good choice.
I'm not sure that the LM324 is considered to be best choice here, that's why I mention it.