Just buy fire rated downlights 240v not 12v because then you have to mess about with transformers, they are not much dearer these days than the cheap crappy things in the contractors pack.
Cut your holes, drop the lights into the holes, making sure that when you cut the holes you will have enough clearance as per the installation instructions from joists and flammable surfaces etc. then get an Electrician in to wire it all up and test. Should be about 1 to 1 and a half hours work for a spark, as long as there is good access above. You could even get the cable if you wanted to save a few quid, but if i did the job i would probably charge you less for the cable than you would pay anyway.
The dimmer will need to be rated for the load you are dimming, and not all lamps are dimmable. Personally i dont like dimmer switches, why not just fit 20 or 35 watt lamps if you want it dim.
Cheers..........Howard