I did not read all the post here, but I'm going to disagree with a few things I did read.
First of all, I've been doing custom tilling since the 80's, I've tilled thousands of acres... I have three tillers that I use, from a 42" to an 80" and they ARE HD tillers, all Howard brand. (rotavators) I've used or seen used many others, but only owned these three Howards.
Most of the tillers, Bush hog, Woods, and the like, are NOT HD tillers in my eyes! They may not be junk but they sure aren't HD!
Rocks the size of softballs as the OP mentioned have NO affect on my tillers, and with MY tillers, it would take a steady diet of foot ball or even bigger sized rocks before I would be concerned at all! I hit rocks and much worse things on just about every job that I do. You may need to remove the stones first, for a light duty tiller, but you won't have to for a HD tiller.
I do NOT agree that a gear drive tiller is better than a chain. This keeps being repeated over and over on this site, but it's not true UNLESS you are up in the size tiller that drives on BOTH ends with gears. A side drive with a chain will take shock loads much better than gears, all of my tillers have chains, and even the first one that I bought in the 80's, has the original chain/sprockets and is still in good shape!
I have tilled ground for customers who have stripped the gears on a KK tiller, in fact one customer had two of them with broken gears. YES, a cheapo tiller will brake a chain, but that's NOT the case with a good quality tiller.
IF you buy a HD tiller and you size it right for your tractor, the OP's stones will not be a problem... AND you won't need to waste time/fuel and make more compaction with a plough/middle buster ect. before tilling...
BTW, you can NOT buy by brand, even good brands like Kuhn makes BOTH light duty and heavy duty tillers, so buyer beware!
SR