On paper buy-to-let appears to be a profitable venture it is imperative to determine whether it works for you, and the only way to do that is to establish your expectations and what you want to get out of it.
For example are you a career landlord? Is buy-to-let your business? Is the profit from going to be your main source of income or are you just looking for a side line to boost your pension?
The easiest way to work this out is to play around with the monthly rental you can charge, research similar properties in your area and what the going rate is.
Find a balance between making enough for it to be a worthwhile investment and being competitive. You can then use this information to calculate your rental yield (think monthly income); divide the net rental income by the value of the property not forgetting to include the costs associated with buying a property.
As a benchmark the UKs average rental yield is currently around 5.9%. Finally think about capital growth, whilst property prices do fluctuate as a long term investment you want to be able to sell the property one day for more than you paid for it.