I would do as much of it with the rotary cutter you have now, and just see how you like it. I have a medium duty brush hog, and it does a pretty good job of making a clean cut, if it's just cutting grass. I'm not looking for a "lawn" finish out of it, just cutting pasture and road ditches. Like Eddie says, you can always sharpen the blades once you get everything else cleared out, and it's down to just grass.
Invest the money you would be spending on a finish mower, towards a decent riding mower, and you'll be set.
Depending on how uneven your ground is, you can use a chain instead of a rigid top link. Also, you should be able to buy a replacement top link, for when you find that your welded one isn't the length you need (like when you get that box blade).