I wonder why you are thinking of replacing the W530? I have just bought one to use as a desktop replacement! I like these quality old ThinkPads too and there is not much better than the W530 in my opinion. If you said why you intended to replace it maybe others could offer more ideas.
If you are not a hardware professional, I would reccommend NOT doing this, because laptops are so hard to work with. You can repurpose the laptop as a desktop by plugging in an external keyboard and mouse and monitor, thats how I did it when the screen of one of my laptops started bugging.
Having said that, you can do that, but dont expect to find much ...
If I opt for Additional 2.5" 2 TB 5400RPM SATA Hard Drive, the battery 4 cells 64Whr battery should be selected.
This is probably because the larger battery takes up the space where a 2.5" HDD can be installed.
Does Using both SSD & SATA Hard drive impacts Laptop Performance?
There is no surprise here, things that run on the SSD run at the ...
It doesnt affect performance, as long as nothing is on the hard drive. If you actively use the hard drive, the app that uses the hard drive slows down to hard drive speed. For some tasks (like rendering a video), you can write to the hard drive without performance issues because the bottleneck is somewhere else (cpu)