Hi
All of the above - and include dieing and drug addicts.
Don't go for expensive homes with a view to getting better clients, even 'up market' tenants can be just as bad as the ones on benefits. Expect them all to give you some problems, there is no such thing as 'easy money'.
Buy cheap, sound, houses to maximise the returns and collect the rent yourself. It gives you the opportunity to check out the property once a month and you will create a personal involvement when there are problems.
Open another bank account to keep a check on income and outgoings for your accounts. Not a business account one - you will get charged. Your name @No2
Always take two months rent up front to cover for the damage when they move out. Not a bond - there are legal requirements to keep a bond in a special account.
If the tenant is on housing benefits then try and 'persuade' them to have the housing benefit paid directly to you. They can sign a waiver to that effect
You will also have to have a annual gas safety check.
Alex