5. the AC coil itself can change freely , like you said , for AC220V valve , if you wanna change it as AC110V , only need to change the coil , nothing else need to change.
here is one more issue , I don't know if you wanna change the DC valve to AC sometimes ? if you wanna do it also . then you can select the RAC coil ( with recitifier inside ) , then you can change freely between the DC and RAC coil then , but as you know , the usage request toward the RAC coil is more strictly than pure AC coil , if the supply voltage is not steady enough , sometimes will cause some problem toward the RAC coil On the user's requirements will be relatively higher than pure AC coil , sometime , if the end user is not very professional enough , may will cause the coil problem due to their inproper operation.
it can say a hidden trouble , please check and consider it and let us know which you want finally.