If you want to make a product, eventually you would need to have it multi-tenant which wnframework is not out of the box.
Also we have not built another product on it for a while (I know there was a company in Chennai who was building a product, but they seem to have abandoned it as I have not heard from them in a while). It is very much possible but you will have to put in some effort to understand it. There is a small note at https://github.com/webnotes/wnframework/
to help you get started.
Another option is to strip ERPNext out of all other functionality (or hide it) except Quotation and then improve it. But then you will be stuck with the GPL license (which is not very bad).
As of now we have very little documentation out of the box to get you started but you can see the ERPNext product to figure how things work. Also be warned that we keep changing wnframework a lot so you will have to keep updating your product too.
Unless you have a lot of time and inclination to learn, I would suggest you should use Django. - If you choose wnframework - we will be very happy :)