Regarding the legs we had the same concern about the plastic legs and checked it out with both Ikea and several contractors. The rated strength for the legs is ~250lbs. per leg. So your typical cabinet will support ~1000lbs. I doubt it will be an issue.
For further reference check out http://www.ikeafans.com. Trust me I was very hesitant based on my experiences with other Ikea furniture etc. but the kitchen’s really surprised me. And they are 40% the cost of other equivalent kitchen cabinets.