The other guys have given you good advice about cutting it.
I would not waste my time with a DR, rent/buy/borrow a real brush cutter on a tractor. Preferable a heavy duty model.
If it were me I would identify what the brush is and spray for it first. You can get herbicides that will kill brush only, and not any underlying grass. A tractor mounted sprayer can often be rented for a day.
The really quick way would be a one day rental of a dozer. You could make quick work of those stumps, brush and even the cars /forums/images/graemlins/smile.gif.
A day rental around here with delivery charge is ~$600, or you can get an operator and dozer for ~$50-100 / hour. The operator option may be cheaper if you have never been on a dozer. Also you will not flip a dozer, with out significant effort.
Once the dozer work is done you would be able to mow, spray, plant easily without worrying about the cars and stumps.
The dozer could also smooth it out for you a bit so that its easir to run up and down with a tractor. The real danger on hills like that IMHO is the hidden ruts, holes and stumps. Your ridding along at ten degrees side angle on a side slope, which is no problem, and suddenly a tire goes into a hole or hits a stump and you can be 20 degrees plus in a hurry. This can flip you over so go slow and keep one foot on the clutch.
Try to go up and down only, but it looks like you have a compound slope so it may be hard to keep it straight up and down.
I like tilt meters, and have two on my tractor. One for side to side and one for front to back. You do not really need the front to back one, but I find it useful to gauge slopes.
Fred