Right off the bat, I have limited experience with VM's. If you are looking to run a 4 slot laptop, you are going to be buying a bigger 15" chassis. The 4 RAM slot laptops run the hotter Quad-core chips. Second, I'm going to make an assumption that many of your VM's will be running same/similar OSes. If so, I think you will probably see the greatest price-to-performance benefit using a SSD Cache since each VM will be requesting a lot of similar data.
Using a an SSD cache instead of a Hybrid HDD will have much higher performance, as the 4-8GB flash in the Momentus XT has been shown to be too small to see the full benefit of SSD caching. Also, if you are really concerned of the single spindle contention issue, you could always swap out the optical drive for a second HDD, and go with an external optical drive.
All-in-All, a Quad-core 15" laptop w/ 32GB RAM, 60GB mSATA SSD, and 2 HDD's is probably going to exceed your budget. If you buy and install the RAM and storage yourself, you might be able to come close to your budget. You are looking at a Thinkpad W520 or similar laptop. SR has a W520 and could probably run some tests for you. I suggest you create a thread in their "Test Lab Previews and Requests" section.