I live in a dark area of the planet, and good headlights are extremely important here. Long winter nights with cold, humidity and wildlife is not a good combination for visibility.
I have restored the headlights on several vehicles with good success. The first time I got one of the headlight restoration kits. I have also had some success, albeit short lived, with Meguiars plastic polish. It works as a stop-gap measure until you decide to do it for real.
Some plastic headlights are coated with a clear coat which will have to be sanded off before you can restore them. The Mercedes was that way. I worked my way through a number of grits up to 3000. I don't remember where I started, but I think it was somewhere around 400, then 600, 800, etc. All wet sanded. Finally a new clear coat which was wet sanded just like regular paint. The clear coat protects the headlights from re-yellowing.
When I restore headlights I like to take the headlights off the vehicle for ease of work. You can tape the surrounding paint work and trim, but I just think it's easier to remove them and work on them on the work bench. This is true even for my Mercedes E-Class where you have to remove the front bumper to remove the headlights. Don't forget to get them aimed after you reinstall them. But if you are going to remove them, it's worth looking at the cost of new ones.
For upgrading headlight output (legally), I have modified both a '76 Cadillac and a '95 Suburban with relays and the difference is incredible. There are ready made kits or you can get two relays and wire one for the low beam and one for the high beam. A search for "headlight relay kit" will set you off in the right direction.
You *can* put HID bulbs in the regular reflectors, but I have found that they blind oncoming traffic more than they increase the actual light output. They seem brighter because of the color, but the beam dispersal pattern is bad. Personal experience/experimentation ended up in the trash.
1989 Winnebago Elandan under "extreme" renovation