I recently configured a ThinkPad T530 with only a 500GB HDD. My plan is the install a mSATA SSD; either 128GB or 256GB later on. The OS will be installed on the mSATA SSD and the HDD will be only used for storage. It seems whatever laptop you plan on buying will also have a mSATA slot for you to install a mSATA SSD at a later point in time. A small SSD cache typically means there is a mSATA slot.
If I were in your situation, I would simply get the 750GB HDD and either 8GB or 16GB of RAM for now. As stated 16GB is only worth it if you are going to be working with very large files.
At a later point in time you can then install a mSATA SSD. Max capacity as of now is only 256GB.