About the closest you'll get to a kit handgun is buying a 1911 frame and going from there. Slide, sights, barrels, components, grips can be had according to your fancy and your pocketbook. The rifle equivalent would be an AR-15 type rifle. The frames for either you'll have to buy from an FFL dealer and the rest you can mail order.
Check our link to the AR15 forum for more info on AR-15 type kits.
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