First when people do this for hard drives it is usually only to grab data from the drive rather than a full repair, that is not to say it is not done but it is not the done thing. Nobody really sells dead drives or boards to the general public and few people will sell dead drives on the offchance and there are a few chancers- if you can get one that has died from a head crash or someone is selling it as SMART has flagged problems get it.
It might be worth looking at one of the data recovery firms but I have yet to find a good but cheap one.
Numbers- ideally they should match but if it is a matter of "my friend has a drive" try it out if it is of the same model or even drive line- different numbers can mean many things from simple bugfixes, lower power logic boards, reworks for component shortages to full on rewrites.
By similar token many drives of the same line can use the same board but with a different firmware or even simple programming- it is rarely advised but it has been demonstrated that some drives are like processors and are built from higher quality/spec components but "clocked" down according to market whims. A quick scan says this has indeed happened (WD caviar of some form I am guessing).
