A 'proper' water injection system pump will run at around 100psi. I forget exactly how much, but the pump needs to run at x psi above boost to be effective. Probably something like at least 60psi. A windscreen washer pump would be next to useless. All that would happen is the boost pressure would work against the pump and you'll probably end up with reverse flow!
I doubt if an injector will be any good either. The water must be properly atomised in the intake system and at the correct amount. You'll probably need a 0.4 or a 0.5mm nozzle jet for starters (depending on boost level). If you don't get the set up right, all you'll do is dump copious amounts of water into the induction system which will simply act as a fire extinguisher. The same effect will happend if the water is not atomised correctly.
As above, a dyno set up is a must to ensure you aren't putting in too much/too little water.
Try searching on the net for a proper set up. I know they aren't cheap compared with your idea, but at least they work as they are desinged too.
http://www.aquamist.co.uk for example has some extremely interesting information about water injection in general. They also make very good kits. I currently have one on my Rover and will be fitting one to by GT-Four in the near future.
Gary