Hi kwendt,
My husband and I sew only with treadles, and we just love them! And what works for us may not work for you, as in budget and what kind of condition the machines and tables are in. I love the “honest wear” of the machines. They've obviously been used and loved, but that's just part of their history.
Is it possible for you to see these treadles in person? Or get better/close-up pictures of the machines, the parts bag and whatever is in the drawers? That way you could see if the missing parts of the Seamstress are there and if there are bobbins for the 9W. You could also see if there will be any repairs or cleaning that you can tackle or more importantly what you don't want/can't do.
You should also take in to consideration your budget and what are the going prices in your area. But if you really love one of them, you should get it. I've never sewn with either of those machines, but from what I've heard from people who have the 9W, they love them.
Where I live in Maine, the $300 price tag is high for an antique treadle. Unless it's a hard-to-find machine, or it's in immaculate condition or it has all of the attachments, oil can and manual. Or it's one you've been searching for, and you really want it.
The most I have paid for my treadles that I use every day, is $150. Two of them we drove to Vermont and NH to get. They are machines/tables and irons that I wanted and there were none to be found near us.
The others were bought here in Maine, and the most we paid was $80. We did have to clean, de-rust and where needed, add veneer. But we didn't mind, because they were machines we wanted, and we love that part of it too.
Three of the treadles we have, we met the son and granddaughters of the person who used them. So, there is a family history with those machines, and we've been given the history of the previous sewist. We've kept in contact with the families and sent photos of the restored machines.
I don't know if this has helped at all, or if I've just been rambling, but I thought I'd comment, if that's okay.
Most importantly, do what's right for you and have fun! They're incredible machines!
Bags
Maine