Mosfets are more efficient transistors especially with higher power.
Also more expensive and used to be in the 80's and maybe still are much more sensitive to static.
Don't just shop at one place. Look around. It keeps the market more honest.
I am not sure if Sparkfun is local to you, that may add to shipping and time.
I have a whole bookmarks folder and sub folders of links to different shops and parts houses.
What you do is pretty neat. My connect is slow so I only watched enough to see you affect light through the slits.
I see how you do it, there is degrees of artistic control in the techniques. Do you also vary the rpm of the disc?
Yes, you are very right to use "house power". Batteries have more associated pollution, recycle or not, as well as cost and charging maintenance.
With the transistor and a controller you can vary or blink the light at different frequencies as well.
I don't suppose that color will come of use? In photoelectric effect, color makes voltage, brightness makes current.
For lenses, find some cheap eyeglass clip-on reading magnifiers or reading glasses and other lenses.
You might also on the side play with polarizing films and other things found in optical experiment kits.