I moved over to mac about a year or so ago and have loved it! But I will try and help you with your questions;
1: Yes working on a 24" is a good choice and if eventually only use the mac then I'm sure you can connect it up for more eye friendly working.
2: If you have an experience with building a computer, or at least a screw driver, I would look into which of the specs can be bought for a cheaper price. 99 times out of 100 you can by cheaper RAM off another trusted website for a fraction of Apple's price. Also its not always necessary to have all the apple branded hardware.
3: Wether you run windows or mac os x on the macbook pro it will still run a lot better than using any pc in my opinion. And if you also look, I'm sure one of the two (parallel/bootcamp) will run in a smaller window within the mac os x. and so you can still run Autocad.
To answer your question on importing into SketchUp, I create a layer of 2d plans and import all the plans into this, deleting on the way the information that has been imported but isn't of any relevance. Once all the plans are imported and in the right positions in reference to each other (more than one layer may be needed if there are a lot of plans) you can then start tracing around and start to build up the model. There is no need to use VectorWorks usually.
Hope this helps...