RS Coyote,
High Flow need will depend highly upon your attachments. The stump grinder requires 17-32 GPM for operation as per Bobcat's specs (Bobcat recommends more than 20 GPM). An 800 series machine (S220 or larger now) will provide about 21 GPM flow max WITHOUT high flow. This would run the stump grinder, but may bog down under heavy loads. The PTO pumps that you are talking about won't need over 21 GPM (http://www.northerntool.com/webapp/wcs/stores/servlet/CatDsp?storeId=6970&N=96+1500). A PTO driven brush hog that you are talking about will most likely only be good for just that, brush about 3/4"-1-1/2". A Bobcat Brushcat has much larger blades and faster blade speeds, so it can handle up to 3" (I've done 4" myself). The 60" Brushcat doesn't require high flow, but the 90" Brushcat will.
The Bobcat Brushcat may be a little more expensive route than the brush hog you are planning, but will be much more machine. The 60" Brushcat is about $6,000 and the 90" version about $9,000.
The high flow option may be harder to find in a used machine. High flow is usually targeted at specific attachments that the general user has no need for. Also high flow attachment are also usually high dollar. I don't know how much the stump grinder attachment costs, but I imagine somewhere between $8,000 and $10,000. Some third party flail mowers (for larger diameter small trees and brush) will run about $20,000. I heard Bobcat will be introducing their own flail mower this summer and should be about $12,000-$14,000.
I don't know how much money there is in stump grinding, but it may be hard to justify buying that expensive of an attachment and a high flow machine. If you are just starting in this business, I would recommend a standard flow machine in the 800 series range. This will give you a large enough machine to tackle the big jobs and about 4 GPM more hydraulic flow than the 700 series machines (S205 and below).
Maybe look at demoing a stump grinder from your local dealer. Try it on a high flow and standard flow machine to see the results. You may be able to buy a standard flow machine and just rent the stump grinder as needed. Make sure your machine has the Attachment Control Kit because the Stump Grinder will need it to operate. If the used machine you are looking at does not have it, you'll have to check how much it will cost to add it.