Many, if not most older 10 speeds had horizontal dropouts. I have a mid-80s Bianchi, an early 80s Schwinn and a mid-70s Trek that not only have horizontal dropouts, but the lugs and other construction details look almost identical. An old frame like that would be good and sturdy and not necessarily all that heavy for what you want. Horizontal dropouts were the norm for old single speeds with nutted axles to help get good chain tension. Frames continued to be made that way even after derailleurs made chain tension a moot issue. When quick release wheels came along, vertical dropouts made it easier to drop those wheels out for quick changes.
If you just want a good inexpensive crankset, without rings, go check out the Sugino AC130 cranks at chucksbikes.com. The only drawback is that he only has 175 mm. On a fixed gear it is considered better to use shorter cranks, even as short as 165 to reduce the likelihood of grounding a pedal going around a corner. Remember, with a fixed gear those pedals are alwasy moving so you can't just keep the inside pedal at 12 o'clock going around the bend. And grounding a pedal on a fixed gear often leads to a crash where a grounding on a freewheeler might not.
If you can pick up a nice Schwinn or Trek of appropriate size for 50 bucks or so you might have your frame and cranks right there. Even if the Schwinn has 27" wheels there is often enough adjustment in the brakes to accomodate the slightly smaller 700c wheel. The pads only have to move down 8 mm (the difference between the diameter of a 27" wheel and a 700c wheel, 630-622=8) or about 3/8". You just have to look at these little details.
All this talk is giving me the itch to finish the conversion of this old Trek to fixed gear. Just picture it. Repaint the frame flat black with black bar tape and a black rack- stealth urban fixed gear commuter. Or should I go really really high gloss black? I just can't decide. I'm just waiting for Rainbabe to get over the silly notion that 3 working bikes is enough. "How many bikes can you RIDE at one time?" Like that's even a relevant consideration.
