There are a lot of hidden cost associated with purchasing a second house, stamp duty, conveyance and legal fees etc and then you have associated cost with getting your own home ready for the rental market while at the same time servicing a bigger much more expensive mortgage. You can't always rely on a steady income from rental property.
Your joint income isn't huge and I am guessing it all depends on your personal circumstances, i.e children/childcare expenses, healthcare expenses, car loan etc. As an alternative would you consider extending your present home?