I have KDE installed on a Core 2 Duo Tower. It runs fine most of the time. About the biggest thing you can get to make that generation of machine Snappy and more livable is an SSD. If you are still running on spinning rust there's no way any machine is going to be considered usable by today's standards.
I don't believe during the core 2 Duo days terabyte hard drives were a thing or at least all that common. I know I was upgrading with 128 and 256 GB drives. So that is very possibly and upgrade itself. But unless your laptop is your main system where all your data has to reside. Or you're going to be using it for mobile video editing? Which would be pretty sketchy with a core 2 Duo. I'd honestly recommend getting a 256 GB cheap SATA SSD toss the two terabyte drive in a computer on the network to share it with the laptop if you need the storage. That's what I've done with the laptop I use. Though it's new enough to have nvme storage. But all my other data is shared over my local network via NFS
Yes. Look up the upgrade process for the laptop. 9 times out of 10 its just pop the bottom off. The other 1 is like the old dell laptop I upgraded. The laptop had to be nearly 100% disassembled outside the screen. The HD was sandwiched between 3 different PCB at the center of the lower half. Bit of a nightmare. The HP, Lenovo, Acer laptops I've worked on we're simple. And the ssd swap made them almost like new. I had a core 2 duo mackbook I upgraded the drive too. Made it very usable.
@Eldritch
Both my Dell Latitude XT, and my Latityde e6440 had ewsy to swap storage. The latter literly had the hard drive slide in in the side, with a standard sata interface. @boredsquirrel