Suspension is generally going to be heavier than a rigid fork, and will soak up a portion of your pedalling effort. However, front suspension isn't too bad in this respect, especially if you can stiffen the suspension up for road riding. It can also come in handy for absorbing shocks from rough roads.

I'd fit some slick tyres as well as fenders and see how you get on. You can get fenders with a reasonable coverage that attach to just the reflector mount on the unsprung part of the fork. You may even have fork eyelets for normal fenders.