There may or may not be a presumption in favor of shared custody in PA. If there is, it is probably very difficult to fight it. You need to consult with a lawyer to see if you have a chance if that is really what you want to do.
You need to consult with a lawyer to see if what H is offering is a good deal or not. If it is, snap it up if you are really going to get D. Time will not make H more generous, but less.
But, depending on your earning power, don't be so sure it is a good deal. He may still owe child support even if you do shared custody, 50-50. See, for instance: http://www.divorcesource.com/PA/ARTICLES/pollock5.html
That being said, once you consult with a lawyer, if it is a good deal and you and H can agree on custody arrangements, then there are do-it-yourself resources for simple, straightforward Ds.
Just be sure to be informed about your legal position before going along with whatever H says.
