I would do the plumbing as it will be a great skill to have and will help with future property assesments and also will give u a good income which your banker will like.  It will also help with improving property as the kitchen and bathroom are where the best improvements can be made to maximise rent.

My advice is dont rush into anything, get interest only loans with offsets so u can park yr money as u save for next property, have a big enough deposit to positive gear each property and dont sell !

Good Luck