Hi,
Would definitely recommend self-propelled ... briggs&straton engine are popular for a good reason (once they are looked after)... try to get a variable cut height in the blades that is easy to do...
Just in relation to the trees - I would recommend putting a border around these, so as not to be near the trunk of the tree when cutting... simple square /circle border of stones or bark around the trunk... especially as the trees are young - cuts & nicks from the strimmer / lawnmower will affect them over time...