Rule number one, you always need more power. The JD 450 is around a 70hp machine with a good reputation. Not a good dozer for clearing trees, but big enough to shape a road or level a pad. If your soil is fairly loose or soft, then it will do fine there as well. If your in heavy clay or rock, it's to small to even break the surface. You'll just slide along the top.
I ran a 450 for awhile and it was too small for my needs. I can take out sapplings, but a tree just a few years old will stop it. Then your digging and prying. It will get our a small tree with some effort, but your burning fuel and moving allot of dirt to get there.
Before you buy anything, be sure to have somebody with you who can tell you what a worn idler or the condition of the bushings. The most common issue on a dozer is a worn under carraige. Next, and probably the most expensive to repair is the transmission. If it's worn or gone, you might not know for a few days.
A guy I know with a D5 got a good deal on it only to have to repair the tranny for $25,000. If it wont move, than you have a very heavy hunk of steal sitting in your field that you can either pay to fix, or let it rust.
My Case 1550 blew a hydraulic pump. Cost $3,000 to rebuild with me taking it out and putting it in. It needs to be redone again and Case wants $20,000 to do it. I'll pull it and have it rebuilt.
I lost a track one day. I have a full sized backhoe to pick it up, put it in my trailer and get it fixed. I was also able to put it on again by myself with my backhoe.
These things happen all the time on a dozer. If you're prepaired to deal with it, than get the biggest one you can.
Eddie