Nozomi, I couldn't agree with you more about how much lighting adds to a layout! From my own experience, i actually took some baby steps to to get to the point where I can solder and add resistors to buildings. There are some LED's available that are pre-wired to accept any voltage (well, usually 8-20w or so) and from DC or AC, but these can be expensive for adding a lot of lights. I used these for some of my early building projects, but the cost ultimately led me to soldering my own, which wasn't so hard with the use of some handy internet tools, at least once I got past my fears of the 'soldering iron'!
This calculator is a great tool for those of us with limited electrical knowledge.
http://led.linear1.org/led.wizYou just enter in your source voltage (which is the rating in watts of the power source for your LED circuit, which is usually identified as the 'output' from whatever transformer or wall wart you are using); then enter the 'diode forward voltage' which will be identified with the LED's you buy and is expressed in voltage (e.g. 3.2v); and then enter in the LED's 'diode forward currrent (which again will be identified with the LED and is listed in milaamps, e.g. 20ma), and then the number of LED's that you will have connected, then ---wallah! --- you get a nice little 'picture' of how to wire it all up!