My car won't start...?

[Annie]

I have a 1997 Kia Spehia RS. We just bought it a couple days ago from someone who said it was perfectly fine but we noticed it didn't have oil so we put oil in it, the next day all the oil was on the driveway. We noticed there was a crack in the crankshaft so my husband replaced it with a new one. We replaced the alternator too & the battery & the cover valve gasket. The car started perfectly fine before & now, it doesn't start anymore. All we did was replace those things & it just stopped working all together. What can it be? The engine is a 1.8L DOHC V6 4 cylinders. Also, is it hard to replace a cam bearing? Is it a doy job or do I need a mechanic b/c the guy who offered to change it for me wants to charge me $600 & we're doing everything on our own b/c we just don't have that kind of money right now. I don't know anything about cars -- please help. :\