Yes, they are rebased DSR's i beleive.
You need to know what K value your other bulb is if you buy a single bulb, but i don't think thety come that way so you'll need to buy a pair
i had a mecca tune kit. 6500k- buying a pair will be fine depending upon the price. however, can i buy a set of projectors, bulbs, and use my existing ballasts/wiring?
the projectors will be hard to get to work since your bulbs are rebased to 9007. I haven't seen any quality projectors with that kind of base to hook up to. there are some companies(riceville town) that sell adapters for this purpose, but they usually have honda and import types, not 9007.
i had a mecca tune kit. 6500k- buying a pair will be fine depending upon the price. however, can i buy a set of projectors, bulbs, and use my existing ballasts/wiring?
Yes you can use your existing ballasts and wiring. EBay sells a variety of adapaters for HID kits. If you buy new rebased bulbs, they will be plug in style that should be compatible with your kit, but it not adapters are cheap. You could even use your ballasts with factory style DS2 HID bulbs.
Keep in mind aftermarket projector lights for Mustangs are not designed for HID lights. They are halogen projectors. A HID projector headlight will be a totally custom job, but will offer better light output and less glare. Also, i beleive projectors use an H2 or H4 bulb, so put off buying your HID bulbs until you decide if you want to run a projector headlight.
Far more information is available on an HID forums...like www.hidplanet.com