the LED's do allow for the dimming function, works exactly the same as the bulb dim,
with the brightness, i can only talk from going from bulbs to blue so going to be less light but with the white LED's it'll have crisper light however it may not be as bright.......not a clue with white so will be a case of try and see,
i think the site i got mine from was:
http://www.autobulbsdirect.co.uk/led-bulbs-12v-c-589.htmlsomething similar to them if not and just a case of finding the right bulbs, i know the front center bulb is festoon but check it's the right size as i remember something about them being an odd length