On the basis of talking through a problem sometimes throws up previously unconsidered answers..........
You have really two separate problems
1) upgrading your system
2) Adding more non-volatile memory
Staying with (2) the hard memory problem and offering my views.
A second hard drive is to me better than one large one on its own. You can then juggle with formatting or replacement if desperate, without losing everything.
The fastest HDs are quick indeed but not all that superior. If you fit one to a slower system it is still available should you upgrade.
Everyone talks in huge HD sizes but few need the outsize ones at the premium prices.
If for security reasons you want to take huge backups away from the computer, then either the expensive (and relatively slow but convenient)external HD, or use the caddie kits which are available to allow a HD to be physically removed from your computer when switched off.