How many stumps vs. how many trees will be left after the loggers are through? Not that this will make much difference, but the only way to justify a dozer is if there are a lot of trees to be knocked down. Either way you will need a large dozer--minimum of a D-6.
Junkman is dead right---unless almost new you will get hurt with maintenance. Also a 6 will also burn 100-150 gallon a day, when let out.
Also, after you knock down the trees they have to be skidded somewhere. Jumping off that dozer to hitch up won't last long -- another machine needed. Then you have to chip them--another machine. Then you need a tub grinder to get rid of the stumps --unless you get a large tub grinder you'll need an excavator to load the tub---2 more machines.
I'm not trying to beat up on you, but that is the way 70 acres needs to be cleared. I once ran a clearing operation for a 90 acre golf course. The equipment used: 1 Timco (a very cool machine) 2 skidders, 1 trailered
chipper, 1 trailered tub grinder, two box trailers for hauling chips, two men with chainsaws, 1 case 580 and 2 men for underbrush, 1 excavator for stumping, and 1 track loader for moving stumps. This took over a month and the the 90 acres was only 25% wooded.
Unless you're looking at this as a hobby you'll never finish (like my back yard) I'd look at the pros. Or better yet, like Junkman said, unless you need the land, leave it wooded.