I personally wouldn't, but I'm a nervous FTM. To me, the car seat is the most important safety item you need, so why skimp? I think it falls under the old adage better safe than sorry. I'd rather spend less money on other items, or skip them completely, and get a new car seat that has high safety ratings. But again, that's just me.
You're right that the carseat is the most important safety item, but taking one that's been treated correctly, is not expired and is from someone you can trust isn't really scrimping IMO. I'll also say that my older two hated the infant bucket, so we only used it for a few months (7 for the older one, 2 for the younger) before switching to the convertible, so saving money there makes sense to me. The place I would go for exactly what i want is the convertible seat since you want the tallest shell you can get in order to keep the kid rear facing as long as possible. If I could I'd gladly by a foonf for #3 and not bat an eye at the $500 price tag since they could RF to at least 4 (my kids are tall with long torsoes out outgrow the RF radian before 3).