It's open to all, but only a few will find the drive to really make a good one. Sorry, but most Blog are empty, boring or are just used as a cheap way to promote something (as a splash or a web page).
I see so many empty blog that are polluting the web. Pages and pages of stuff that have not been updated in years. What a mess.
If you think that you can take care of a blog, then do it. But if you don't have the energy to write a few pages a week, then don't. It's the simple truth.
You can have guest on your blog, it's great. As long as you are writing more then your guest... It could be a good idea to meditate on the reason why you have a blog. Maybe, the blog is not in your goal anymore. And if so, you should really think about finding a way to get some new content with the help of guest writers.
I saw people spending money on blog that are filled by paid writer. You should spend your money on a book instead of a blog. Because a book can be sold for years and years. A blog when it's becoming outdated, it isn't worth as much.
A blog is something for people who likes to write. It's simple, if you don't like to write then don't open a blog.
Yes, it's very nice to have a private domain and your blog on it. It looks professional. But if you don't fill it with good stuff, you'll be throwing money by the window. You should start with a free blog first. Then after a year, if you think that you can keep it fresh and up to date go for your own domain.
I remember when the first blog were popping on the web. It was all about people who were writing about their life. Simple day to day stuff. The ones who were good enough to write one post a day rapidly became stars. Now, people are opening a blog just because they eared that they could make money with it. Yes you can make money with a blog, but you need to have a great blog with a lot of traffic. And you need to come out of the sea of blogs out there on the web.