I wouldn't describe a bike with a non-functional first gear as being in excellent condition.
Yes, you can start in second gear in many situations. There are times, however, when first gear will really come in handy. Think about it, if it wasn't necessary the manufacturer would have saved the cost of the extra gear!
What happens when you try to shift to first? Does it just not shift? Does it find neutral instead? (I absolutely hate it when my bike mis-shifts into neutral when I down shift climbing a hill, almost guarantees I don't make it). Does the transmission lock up?
If something is just messed up (bent) in the shift mechanism then it would probably be okay to continue to ride it. On the other hand, if the problem with first gear is that something has broken then there could be loose pieces rattling around in the transmission which could jar loose and end up in a really bad spot, causing even more damage.
If you approach it from the perspective that you are going to immediatly tear it down and fix the transmission then I would say it would be an okay deal. You would certainly learn a lot and would end up with a great bike with a fresh bottom end at good price.
If you don't plan on fixing it yourself then I would not recommend it.
Rod