Sounds very nice, and sure to be ideal for someone, like you say even a small adult. I ride Dales and Fell ponies quite happily.
There are a lot of horses and ponies for sale at the moment round here. A lot of the livery yards have put prices up, and the increases in feed, etc. has made some people realise its far too expensive and they are selling up. I notice in my local feed and tack shop there are dozens of ads either free long term loan, or even free to a good home.
As I see it, they are not broken in, and are in foal. OK, you are getting a free foal of course, but its another mouth to feed, and you cannot do anything with the mare until she has foaled, and then the foal has been weaned. So, in pricing that has to be taken into consideration.
As to how much, well, I would price her a little higher than you are thinking, and then you can always come down. See what reaction you get. Suppose it just depends on who is looking at the time.
Having said that, coloureds do seem to be in demand and sell, so good luck!!