I'd save the hassle of 3D and alpha blend a sprite, I don't think it would slow your engine down as much, but maybe a full screen hud is a bit ambitious. You could have a top and bottom hud.
One small problem, it's quite hard to get anything to appear on top of sprites, so if your using sprites, use sprites for the little bits like dials, digits, crosshairs etc. It's really easy to rotate sprites and scale them, so you should be able to do something cool with just sprites. You can change the diffuse colour too, so if you wanted to go night vision for example, you could change the diffuse colour to suit and it would recolour the sprite.
Van-B
My cats breath smells of cat food.
