I have a Northwest somewhat like the one shown for about $150 that is supposedly accurate to 1.5mm. See if any at the URL will work for you? Needing one to work outside is the tricky part; make sure you get one bright enough as the glasses only help so much. Look for one that will calculate triangles to give you the length of one side. Just another way of tweaking accuracy to what you need. I haven't shopped for one in awhile and am pleased to see prices have actually dropped some.
I used my digital measuring things mainly to get square footage indoors or to do quick basic floorplans and I did not require the accuracy you do. Nothing against lasers but also look at sonic and even GPS measuring devices. They use GPS now to plumb skyscrapers.
Please post what you decide to get. I am curious.