Gary,
I have a slightly different view than my respected DIY friends above.
The first pond I was involved in building was with a "pop'n johnny" and a hand operated scoop. Dad was on the tractor and guess who was on the scoop?
I would never say a small dozer is worthless. I would say that a small dozer has limitations, but within those limitations, is an excellent machine, and a far cry from the old hand scoop.
You wrote "I need to maintain my fence lines and do light clearing tasks." I have a Case 450C that is absolutely perfect for such tasks. I wouldn't own land without it.
Want to build a 10 acre lake? Limitations. Want to clear out virgin forests? Limitations. Need to move large volumes of clay in a short time? Limitations.
I'm currently clearing 70 acres of the thickest 5 year old clear-cut you can imagine. The 450 is perfect for my needs. I can take out trees to about 8 inches which is the prevailing size after 5 years.
I've renovated two ponds with the 450. You can see the dozer in action at this link which is the renovation of a 1/4 acre pond into a 2 acre pond.
http://www.meadowlarkponds.com/walden.htm The real beauty of the 450 size machine is that it can be hauled around on a flat bed trailer and a pick-up....as opposed to large 18 wheel type trucks/trailors required for larger machines. It can also be worked on by just about any competent tractor mechanic. When mine has a problem I can't solve, I just load it up and take it in to my tractor guy who can fix it. Parts are expensive, but the labor is the same as working on my tractor.
I'm not familiar with the Mitsubishi BD2 or the Komatsu D20 and my biggest concern would be availability of parts and expertise to work on them. As Eddie mentioned, they are high maintenance items and if you can't get parts and/or someone to help you if/when you have a mechanical problem beyound your capabilities, then the machine would be worthless.
If you want to operate mine, see the results of fence clearing, see the results of 70 acres of land clearing, or see ponds, you are welcome to visit Meadowlark ranch.
One final thought...safety. If you do decide to purchase or rent a dozer it is helpful to have an experienced operator kind of show you the ropes. You can get in trouble on a dozer...just as you can on a tractor. They are unforgiving.
Would I prefer to have a D6 or D8 size machine? Depends on the tasks and job requirements. For larger ponds, for moving large amounts of dirt in short times, yes absolutely a larger machine is more appropriate. That kind of machine I would probably rent for short term needs. For day-to-day property maintenance, fence clearing, light clean-up, pond maintenance and small pond building, and hurricane recovery, I love the 450 class machine.
