Hello and welcome.
I will tell you that I personally am against spanking, so I can understand the guilty feelings you are experiencing.
I think the first order of business is that you and your wife need to get on the same page in regards to discipline with the kids, otherwise you will be fighting a losing battle if one parent is not backing the other.... this of course includes consistancy and follow through. If they know that you will ask them to do something five times - they will wait for you to ask five times, if they know that you will threaten a consequence and not do it - why listen?
Instead of fear, I think the key is respect.
(not the same thing)