Jeff,
When I bought my land and started clearing it, I had the use of a John Deere 450G dozer. At first, I was pretty excited about all the land clearing that I was going to be able to do with it.
I found that it was fine for taking out small trees, but was pretty useless on anything over 6 inches thick. I could get them out if I removed enough dirt, but it took awhile. The larger the tree, the longer I spent messing with it.
Then the real shocker. Moving those trees to a burn pile and cleaning up the mess was just about too much for that dozer. It just spins its tracks when trying to push just a few trees at a time, or a pile of brush. I actually gave up and went to chaining them up and draging them with my backhoe and 35hp CUT one at a time.
I did have allot of fun heading off into the jungle and cutting new trails. As long as I stuck with the small stuff, It was great. Next to finish work, blazing new trails is my favorite thing to do. Then the prolem of cleaning up those trails became apperant. It was just too small a machine to push out piles of debri. I'd start with a small pile, but they tend to grow on you. The farther the push, the worse it got. Every time, I'd have to stop, split the pile in half, and push that half. It just wasn't practical for getting anything done.
The final straw came when I was digging my small 3/4 acre pond. I had started out dgging it with my full sized backhoe. It's a Ford/New Holland 555E in 2wd. I would dig out some dirt, then haul it out with the front bucket. When the Deere 450G arrived, I thought that I'd just lower the blade and start pushing dirt. It was too light to dig into the hard packed clay. I tried the edge of the blade too, but it wasn't able to break through.
A small dozer like that needs rippers to break up the dirt before it can push it. That dozer didsn't have rippers, so it was useless in pond digging for me. Even if it did have rippers, the blade was too small to move very much dirt anyway.
It was a nice sized dozer for shaping and smothing out the shorline of that pond. With loose soil, it's ideal for this. Another good thing about that sized dozer is when you get it stuck, it's easy to get unstuck. I burried the tracks in mud while working on the edge of the pond and was able to pluck it out with a chain attached to the hoe bucket of my backhoe. The backhoe has plenty of power for pulling out a dozer of that size.
Having said that, Meadowlarkponds is a member here and he has a Case 450 dozer. He loves it and has cleared allot more land then you plan on clearing with it. He's also dug at least one pond with it, but I don't know the details on wethere that was the only tractor that he used or not. He did have his last pond hired out to a crew with two D6's, so that might say something about small dozers too.
When I decided to buy my dozer, I wanted something allot bigger then that 450G. My goal was low 100hp range. I looked at a Case 1150, Cat D5 D6 D7, JD 700 850 and the one I bought, a Case 1550. I paid $25,000 for it and have put about $10,000 into it in repairs. It's a 1988 machine with allot of hours on it. The meter isn't accurate, so there's no real way of knowing for sure. It's big enough to take out most trees up to 18 inches by just pushing them over if the soil is soft. In hard soil, middle of summer type conditions, I've had issues with 12 inch trees. Soil conditions make a huge difference in what you can push over and what you have to dig out!!!!!!!!
I also found that digging ponds takes allot of time. My blade is rated for over 3 yards. That means that if I lower the blade an inch into the ground, it's full in about 20 feet. The rest of the distance that I'm moving is just travel time. Going back and forth all day long to dig that one inch in 20 feet takes hundreds and hundreds of hours to dig a decent sized hole. My blade is also 12 feet wide.
Clearing roads and trails with it is still allot of fun. Unfortunalty, cleaning up those trails is still a nightmare. The problem with a bigger dozer, is that you make a bigger mess. With 168hp, I can really twist up some trees into a tangle that takes months to undo.
Last year I tried something new in my timber clearing techique. I used the bachoe to take out the trees, then drag them to the burn pile right away. I kept the road clear from the very beginning and kept it up on the entire road. There are four roads in that area that are the same size and distance from the burn pile. The first three were done with the dozer, then cleaned up with the backhoe. Each of them took a day to clear with the dozer, but months to clean up. The newest road took two weeks to clear with the backhoe and it was done.
I will never clear roads or trails with the dozer again. It's just not effective when adding in the time and effort it takes to clean up the debri they create.
My dozer is now just used for shaping and spreading dirt. It's worthless for moving dirt very far, and it uses about five gallons of diesel per hour to run it, so I'm much more concious of when I use it and what I use it for.
To move dirt, I load it into my dump truck, drive it to where I want it, and build up massive piles. This might take a few months, but then I spend a few hours on the dozer to shape and spread it out. Fuel wise, I'm saving allot of money. Work wise, I'm moving along at a fair speed, but it's all progress. No more doing something, then having to clean it up again.
As for the ideal size, I'd think something in the 100hp range with a six way blade and hydrostatic drive would by ideal. Gear drive will get it done, but stearing and changing direction with hydrostatic is a huge plus. That Deere dozer had a solid sprocket, which meant pulling the tracks to replace it. Most other dozers have five piece sprockets, which make changing them real easy.
If Cat is the closest dealer to you, then you should seriously consider buying a Cat. One fact is true with owning a dozer, it will break on you. They take allot of abuse and they are very delicate machines. Trees and brush clearing is probably the most abusive thing you can do with a dozer. I've had tiny pine trees work there way through my belly plates ant take out hoses, filters, wiring and my oil sending unit. Never drive over fallen trees!!!! Push them aside and always work on dirt.
The Cat D5C is a low track dozer that's a good machine and probably in your budget. The high tracks are the big money machines, and more then you really need.
I would only buy a machine that had a dealer close by. Do a search to make sure, but if Cat is your only dealer within an hour of you, that's really all I'd look at. Cat might be the most expensive, but they have earned that right with the best support and a history of making quality machines. Since you plan on keeping it for 20 plus years, think support first. It's the most important part of owning equipment.
Eddie
But you go to the mid size dozers adn they JUMP in price. I have a 1993 Komatsu D37P here that will sell for low $20's when completed and this is still only 17,000 pounds. OF course when you are operating a dozer doing yoru own work you will have a nice warm fuzzy feeling that is worth all the hassles. 
