Well, there is no advantage or disadvantage if you do so. However, if a directory shows sites in alphabetical order, your site would show up before the 'A's
Nothing like that. Domain name should be short/easy to recall so that branding can be done. For SEO purpose you can use long keyword rich domain names.
As eddy2099 has suggested, the only advantage of a domain starting with "123" can be in a directory ordering. That if ordering is done by name, of course.
I don't like using numbers in the domain names in general. Read this for some hints about choosing a domain name: Some hints for choosing a domain name