If you go with a Mini 14 I suggest you buy the 'Ranch Rifle' as it comes drilled and tapped to accept a scope. Buy a newer model if you can as they appear to have improved accuracy over the older ones. Be prepared to put up with poor accuracy with the older models, but there are ways to remedy this to a certain degree, and some of them actually shoot reasonably well ... from what I've heard, mind you.
I think it would be helpful to lay things out and try to keep comparisons fair.
Mini 14, used .... $500 to $600 , new $900 plus tax or about $1000
AR180B used ..... when I've seen them for sale, they are about $1000, new $1075 plus tax. Add $175 for a rail if you wish to mount a scope, which brings the new price to $1250 plus tax or roughly $1400.
Rob Arms dosen't seem to be available at them moment, though occasionally used models show up just under $2000. Rails, etc are extra.
Used SL8, around $1800, new, I'll trust the poster who says $2500 as I can't seem to find a listed price. Add $200-$300 for tax of course and a couple hundred if you want to use AR15 magazines in it.
If you can do without a scope, the VZ858 isn't a bad choice at $650 plus taxes new or $550-$600 used. They are accurate, can use inexpensive milsurp ammo but I have yet to see a good solid scope mount which will work on them (so I wouldn't buy a VZ if using a scope is paramount).
I like the idea of starting with a budget first, then buying a rifle within that range.
Bear in mind that you can sell your rifle off and buy a better, more expensive one down the road. In the meanwhile, you get to shoot it.