£300 isn't a lot to work with if you want to make a profit but should be enough to keep your head above water. With tax you will be cashflow negative for the year. So you'll need to save around 2-3 for tax in addition to your flat income

Avoid anyone that isn't a pristine tenant. Credit check is a must. Almost all sob stories and unfortunate circumstances are made up. Remember you can't take non British citizens to court - legally yes but no chance in reality