good established neighborhood

I buy a piece of nice land...build..and then within a month there junk all over the nearby plots...for sale signs and other crap....you know the drill...

Can anyone suggest a place on the mainland that is established (meaning neighbors have been part of SL for a while) and there isn't a risk of having crappola pop up around me?

There is no such place. That is the risk of mainland. You don't know who will be your neighbor or what they will put up next to you, unfortunately.

There isn't a place that is guaranteed 100% to be the things you want on the mainland (as opposite to private regions with zoning rules as part of covenant).

That said, if you are willing to pay more per unit land area, that often is a way to avoid such issues or at least to reduce the risk of it. A lot of "crappola" pops up on cheaper land, some of the more expensive plots often "price out" the creators of such things (though as I said, this is not a guarantee). From personal experience, the continent which contains the "Nick Rhodes", "Warhol", "Cezanne", "Picasso" etc sims fits the above description. I've been a longtime resident of two of those sims and definitely have notices less of the problem that you described, compared to other plots I've had on the more traditional mainland.

Or, buy plots in sims which only have a smaller number of larger parcels rather than a larger number of smaller parcels. Again, this is likely to be an expensive solution.

Another trick is to try to buy a plot which is surrounded (or at least bordered on a few sides) by Linden protected land/sea.