whats the best HDD I can get for the laptop?
You have to use an 2.5” SATA HDD.
SATA controller is not limited to an specific size…
The question is what HDD size is supported by BIOS… but from my point of view you should be able to use 500GB HDDs without any big troubles…
Furthermore there are HDDs with different speed: 5200rpm or 7200rpm…
Using 7200rpm HDD you'll get a litter faster access to the files and faster booting but the difference is not really noticeable…