The render surface can be a backbuffer or an offscreen buffer, just as you want it to work. You can use Flip if you create your surfaces with one call to CreateSurface (specify COMPLEX FLIP 3DDEVICE and PRIMARYSURFACE flags in DDSCAPS). Or you can make a primary surface and an offscreen plain surface. The offscreen plain surface then has to be created with the 3DDEVICE flag and will be blitted to the primary surface using Blt or FastBlt. Just as you want.

To mix 2D and 3D just simply use the backbuffer to Blt on. You can Blt anything what you want but beware that you first Blt your 2D surfaces and then render your 3D stuff. Otherwise your 3D things won't be visible because they will be overblitted by your 2D surfaces.

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