No oven proof stew pot, could I use a metal saucepan with instead?

While the metal saucepan may be "safe" to use in the oven, you might not get the results you would like. The saucepan is probably a lot thinner, lighter weight material than a typical oven "stewpot". This may cause the outside edges of your dish to be very dry, crispy or even burnt. Most stewpots are very heavy duty. I could leave something in the oven cooking in mine all day and not worry about it.