If you want to start a professional blog without buying a hosting then the best place is blogger.com. As you want to monetize your contents through google ads and want a custom domain there no other choice better than blogger.
Free hosting accounts are there but relying on it will lead you to buy a hosting later whenever your website gets some few traffic. So it is not advisable. And about WordPress, you can't add any ads of your own unless you get an upgraded WordPress or a self-hosted one. There are many other blogging platforms too but you can't have a custom domain with all of them and even if you can do it there may be any limitation such as adding a code or placing an advertisement on it. Above all of these, you must consider the uptime of your blog and the security of your data. Here with the term security, I am not pointing a firewall but any data loss that could happen with non-reliable platforms. Blogger has an easy backup option that too as a small XML file.
All the above limitations can be sorted out with an ease of using a blogger blog. I suggest you go for a custom domain as in your question instead of using a blogspot subdomain. Recently blogger has made an option to use https on a custom domain. So you will get a not only a free hosting but also a free SSL. Isn't it good? Blogger has many new attractive refreshed templates now. These themes are super fast and very responsive on almost all size displays. You are also getting an out of the box SEO on new blogger. Pointing your domain to your blogspot is also very easy. There is step by step instructions provided on blogger. Using a blogger platform from google you can fully concentrate on your blog and you don't need to worry about updating or reinstalling plugins, updating platform version, handling spams, security checks or anything like bandwidth, traffic limit, inodes, entry process limit etc. All these will be done for you by Google for free of cost through their amazing platform. Why shouldn't use this feature? And why should you go to another platform?
If you are planning to place google ads you can just add google ad gadget there on the layout if you have an already approved Adsense account. If you don't have one you can apply for an Adsense account right from the earning tab on your blog's dashboard panel.
While Google is giving such a beautiful platform to work on with then why aren't people utilizing it? People have a mentality that something that getting free is to just play with it or to make an experiment. Several spammers are also using this platform that will give others a feeling that blogger is not for me. Many people just start with blogger and they will lose their efforts like nothing and then they blame on blogger for not getting traffic. But they are not realizing that their success in blogging lies in their content than a platform. If you use blogger wisely then you can be a successful pro blogger for sure. If you have any doubts you can ask me here for comments.