For a start you can check out this thread although it's a very long ongoing thread. It's full of great pictures and sometimes ideas on pricing
http://www.quiltingboard.com/vintage...s-t130994.html
I think from what I have read treadles have been made since the late 1800's up until maybe the 1940's and 50's so there are lots of choices. From what I can tell since the White machines weren't changed as much you can find the parts and pieces if a machine needs help. If the White serial number starts with an FR then it will take one of two different bobbin cases and they are supposed to be easy to find.
Also since there have been so many Singers made they are easy to find as well.
I have found that just reading through the postings on this board and doing some online research you will be able to make a good choice when you are ready to buy.
Good Luck