When dry give a good squirt of light oil, directly into the lock via an aerosol extension pipe, or put some light oil onto the key and waggle it into the lock rinse and repeat several times all locks, do this regularly as washing the car with (especially pressure washes) detergents removes the oil.

Mudflaps depend on why you want them and how you drive, most cars have plastic rear bumpers now and plastic inner arch liners so won't save much rust wise (but could arguably help stop stone damage and muck spatter along the sills with front flaps), if it's to stop excessive muck being sucked up the back of the car then sizeable rear flaps are called for...then your driving must be very good because otherwise if you reverse up to touch stop or reverse over highish kerbs the flap will be trapped and ripped off sometimes causing other damage.

You shouldn't have to drill any holes, most come with fiddly little clamps of varying quality, dealer genuines whilst usually more expensive are usually the best bet, just like floor mats.

Edited by gordonbennet on 23/01/2015 at 13:27

The best way of dealing with frozen door locks is to put the key in your pocket for 10 minutes before use - you'd be surprised how that warmth will make a difference.

Or ... you used to be able to get small cans of de-icer, which had a nozzle suitable for doorlocks. No idea if you still can.

Lastly, if you do drill holes for mudguards, make sure you rustproof VERY well.