Nowadays, it's common practice to use an SSD for the operating system/apps, and an HDD for data storage (in desktops at least), with laptops, it's best to use an SSD, and an external HDD for storage. Anyhow, I don't see why you should make any compromises.
I'm sure you could easily find an I5 laptop that comes with an SSD.
Depends on what you'll be doing, but SSD's give a huge "real world" performance benefit, you'll definitely get benefit from that, and you only "might" get benefit from faster CPU and more memory... i.e. if you're doing some really intensive stuff.