Hello,
Thats a huge question I must say, I will just answer it in a way that it will cover all your question.
You can not start a business with a name in which already there is a business, even if the said person has not operated the business since last many years.
The law is you can not use a name which is deceptively similar to other name. However if you start a business with the same name then you might face a suit to defend.
If a person has not registered the trade name as trade mark then same will be alloted to you if no objection is raised by them at the time of filing.
Logo with name is the best way to protect your mark, it is usually filed in jpeg format.
Trademark protects your brand in n number of ways, to give you a brief it is a way in which you protect your business so that no one else in future uses your brand identity.
Let me know if you have further query.
Regards