Hello.
So as a part of my profession, I build lights for cars. And honestly it was really hard to resist for me to install projector inside w124 lights as I did not want to spoil the classy look of w124 with a retrofit, even though having collection of some of the best hid projector produced till date, and things I know at the back of my hand.
On to the topic
The previous owner installed the 100w halogen but still it was hard to resist so I installed hid but with a custom made casper shield, To prevent as much as glare I can in low beam. Its way better in output than 100w halogens,maybe because I am using true 55w boosted hella ballasts.
Although glare is very controlled but you still cant have true cutoff that of a halogen bulb.
If you really wanna install hid then you can get a good bulb from Morimoto or directly from China. As no oe brands likes of Philips, Osram make kit/h series bulb.
For ballasts get a boosted Hella ballasts. They are basically 35w but can be boosted to 38,42,46,50,55w respectively. And no aftermarket ballast matches it in quick startup time. Aftermarket you can get Hilux, morimoto ballasts.
Relay from Morimoto.
Hope it helps