Tough one to answer really as it depends on what your loan repayments will be on completion.
As for earnings, well, that's almost impossible to answer too as it depends on many variables such as whether your flying club would pay you a retainer and their going rate per hour for instructors.
You can only really hazard a guess at what your training will cost you too as you may need extra hours due to unplanned hiccups which every student pilot has experienced at some point to a greater or lesser degree.
Check out some of the threads like "worst paid instructor" and other threads started with similar questions to yours in the last 6 weeks on this forum.
Sorry I do not sound too helpful but it really is an unknown qauntity. My advice would be:
Unless you have back up financial plans I really would not advocate relying on part-time instructing towards your income as it is unreliable due to nature of the industry and the soddin' British weather.
Hope this helps,
VFE.