you didn't indicate which MS you wanted to be
There is only one kind of Merit Source. There is no split between "global" and "local" ones. Each source simply has N additional merits to be sent per month, and that's it. People are not restricted to send merits only to the local or global boards.
ones you're a Merit source all eyes will be on you
It doesn't always work in that way. There are some sources, which were discovered after a year or so, and where many people still don't know, that someone is a source, unless they notice, that the amount of sent merits is more than half of the received ones.
It is true that as a merit source you will have the areas or boards you prefer to give merit more and the kind of posts you will be meriting more.
Obviously, technical boards are overmerited. But on the other hand, they are actually pushing us forward, so it should probably stay that way.
It good to have quality posters drop application for merit source than leaving it in the hands of the worse to be applying, just as I have seen in recent applications.
There are some sources, which didn't create any application. Some people just open their inbox, and see a PM from theymos, telling them, that they will now keep receiving N merits per month.
users who are among the top 500 based on bpip ranking, I personally see a problem in this
It is hard to find great posts from newbies. And even if you find it, then it is usually some alt-account of a higher ranked member, who is just starting yet again from scratch, after being Legendary for the Nth time.