Halogen bulbs would not fit in in a good HID projector. The bases of the bulbs are different shapes. It would be a huge waste of time and money to do a retrofit and keep halogen bulbs. All halogen bulbs are garbage. I am one of the people who will never spent a dime on aftermarket HID systems. I just don't think the quality is there, but to each their own right? If you're concened then get an OEM setup. With OEM ballasts and OEM bulbs it will be far less likely you'll ever have any issues as compared to aftermarket systems. You could even go as far as making your own wiring harness to use quality parts and make it fit your vehicle perfectly. It gets expensive fast going this route though. I spent about 2-2.5 times more than the average retrofit so if you are concerned about cost then this might not be the option for you.
I just finished mine this past weekend and the Denso ballasts were easy to hide. You can still access them if needed without taking the front end apart as well. They fit under the extra battery tray and air box tray. I spliced in a set of AMP connectors on the Densos so that I could put the D2S connector in the housing and run the wires out of the housing underneath so that no holes were visible before putting the plugs on the wires. My housings have a single 1/2" hole on the bottom of the housing instead of going straight out the back of the dust cap. The stock dust cap also fits on it with no modification. I also created my own wiring harness instead of using the typical Morimoto harness. The reason is that the premade harness has too many connections that are points of failure in the future plus it didn't fit well. Wiring should fit the car, it shouldn't be the car that has to fit the wiring. The premade harness had 3 separate grounding connections, the capacitor just flopped around on the harness, there were adapter plugs, and some of them didn't have the (+) wire on the correct side of the plug so you'd have to plug things in backward.
Anyways, I consolidated everything down into just 1 ground and 1 positive that go to the battery, bought quality parts/wiring/braided sleeving, and put the capacitor inside of the harness so that it doesn't flop around and is streamlined, and also don't have any adapter plugs now. It wasn't difficult, just tedious and time consuming...although I'm also extemely anal with wiring being perfect so that always adds time for me. All the wiring is nearly hidden as well and it just fits the truck a lot better. By doing it this way each headlight is its own independent circuit and both are driven off of the single (-) and (+) connections.