RE: Clearing land help
Personally I'm just to lazy. I would hire it done. In my area there is always a guy needing money that has heavy equipment. No harm in calling them all and seeing who is the cheapest. This method has always netted me good results on work done. Seems that a man that needs the money more to provide for his family does an outstanding job. He is depending on you recommending him to others to help him out.
On leaving the trees. The roots usually are about as wide as the branches, they call it the drip line. I put a plot 5' or so past the widest branches. That way they aren't competing as much. And only beneficial trees are left.
An old trick with the trees is to saw or chop a circle around them an inch or two deep. The circle must connect. The water travels just under the bark. If there is no route for the water the tree dies. Stumps can live for a long time before rotting when a tree is felled. Kill the tree first then chop it down if you are going to do it yourself. That way the roots will rot quicker. If you did it this time of year you would have to wait 6 months or so to make sure they are dead.since it is dormant and everything is in the roots. If you wait until they leaf out then do it there is more of a strain on them, they will be dead in a week or so, just wait a full month before you fell them.