Disk drive (failing) question?

I have a computer, purchased in 2007 (or was it 2006). It's a Dell Dimension C521.

It is slow, compared to newer computers. I am mostly using it as a test machine. If it dies, I can manage without.

Recently, when I powered it on, the BIOS message said "No operating system found".

Naturally, I tried CTRL-ALT-DEL, and it booted. Running opensuse 13.1, I had one instance where a message was logged saying that the disk response was slow. Apart from that, it has been fine. But, if I power down, the same problem will arise.

From this, I conclude that the disk surface is fine, but something else is failing:

1. Possibly the SATA controller on the main motherboard is failing;
2. Perhaps the device electronics is failing (on the disk);
3. Perhaps the disk is slow to get up to full rotation speed (congealed lubricant?).

For either (2) or (3), a relatively inexpensive disk drive replacement would solve the problem. For (1), it might be more trouble than it is worth.

Perhaps somebody has similar experience, and can tell me which is more likely to be the problem.

And if I do decide to purchase a replacement disk, do I need to worry about SATA II vs. SATA III? Or will a SATA III disk do just fine?

The old disk is 320G. I would probably replace with 500G. Going to 1T might confuse the BIOS.