Firstly the phone is not known for any common problems (although the keypad has been critisised as being too small) and the phone is well made and can take quite a beating. IF instead you mean lifespan as in battery life, it all depends on how you use the phone. Keeping it in flight mode solely for media usage will bring you about a week or so, and keeping it in phone mode will give you 2-3 days (based on my usage).
As for the price tag, I wouldn't say that's too good to be honest, as here in the UK, you can buy a K800i new for £65 (probably equivilant to about $120), so I suppose $100 would be ok, but you wouldn't be far off being able to afford a new one, so I'd suggest getting it new if you can find it at that price.
The K800i is a really good phone though and it has a lot of features to enjoy. I hope this helps
