regarding HD movies - that need space and processing speed:
USB 3.0 drives connect with 5Gbit/s to USB3.0 drives.
or - even faster - Intel's Thunderbolt connects with 10Gbit/s to
Thunderbolt drives.
Both technologies by now have hard drives and RAIDs on the market.
They let you work on your HD movies while these reside on the external drives. RAID being preferred in your case for faster read/write access than single drives.
So, even if you buy a laptop with a small-ish SSD you can still work on Terrabytes worth of footage.
It lets you choose a much less expensive lappi if you can live with having to work on an external drive.
If your old Dell already has USB 3.0 then a USB 3.0 RAID is worth a try to give you an idea of what a new lappi really has to have in terms of CPU and memory.
I only know of Apple Macbooks that have Thunderbolt built in.