Plunge routers are ideal when you are starting in the interior of a project, yet molding is generally carved on the exterior side. If you want to make fancy molding with interior beadwork, a plunge router may be a little easier, yet it isn't necessary. If you show us the type of molding you want to make, it would be easier to make suggestions. You also want to consider whether you want to move the router over the lumber or the lumber over the router (using a router table). You'll probably get more precision from using a router table, and a fixed router is ideal for that. If you're not experienced using a router, practice on scrap material before taking chances with expensive lumber.
