The best thing to do if you're serious about getting a Ranger knife & have questions, is call. I found it easier & more informative than through emails.
The standard RDs are very thick & are not really great slicers, however, I would think a RD9 would be wanted more for its chopping abilities. If not, Justin can & will put a grind on it that would be more suitable for your needs. I believe he's also used thinner stock for a custom, too.
I found that the RAK (Preferably with scales on it), is a great compromise. 3/16" 1095 makes for a better slicer than the RDs & can still chop.
I've always thought that a RD9 & a G-10 or micarta scaled shiv or RAK are a great combo to have & would end up costing you just a little over $200, but you don't have to do it at once.