i rented a 6 ton mini last summer thinking I'd push trees with it. I was able to push trees over up to about 8" diameter, and that took some work. I'm a novice, but i think I had a decent grasp of what to do.
I ended up paying a guy $1000 to push my 30 trees with his track loader. Well worth it.
I then bought a used backhoe for $4500 and moved over 400 yards of dirt. If you go this route, replace all hoses and fix all leaks before beginning. I can't tell you how many hoses I blew before I decided to replace them all (about 90% really). It's so much nicer not having to worry about them. A few small leaks mean I have to add about a gallon of hydraulic fluid for each day I work with it. I've had to purchase tires, rebuild cylinders, fix the brakes, so figure that into your equation when you look at older used equipment.
I wanted a mini X, but I'm glad i didn't buy one. They would be handy if you had a skid steer though.
By the way, a lot of guys prefer 2wd backhoes because they weigh less. I've gotten mine (2wd) "stuck" a few times in fill dirt that I moved, and actually thought I wouldn't be able to get it out. Very stressfull sitting on an incline sideways in a 12,000 lb machine. Usually lifting up with the stabilizers and putting something solid under the tires is what gets me out, but seems like that's the last thing I think of to do.
Only buy a used machine if you're willing to learn how to work on it and willing to buy the proper tools to do so.
Oh yeah, and backhoes do **** at grading. You pretty much have to dig it to the grade you want and backdrag or scoop it up with the loader bucket. At least that's what my novice operating skills tell me.