Based on what should i choose an amplifier to implement a Howland current pump with?
You can use this circuit, which is better than a Howland source unless you really need one side of the coil grounded. 







simulate this circuit &ndash; Schematic created using CircuitLab

You may wish to put some diodes on the output of the op-amp to either supply rail, and put hefty enough bypass capacitors that the stored energy in the inductance can't over-voltage the amplifier. 

40mA is pretty easy, you could also just use a regular op-amp and a couple of emitter followers (PNP/NPN) on the output if you are not concerned about crossover distortion. If you are, then you need to bias the transistors somewhat 'on', and there are many schematics available of op-amp boosters.