buffer and buffer spring?

The you can use the M-16 buffer and spring if you are using an A2 fixed stock. If you are using a m-4 style stock then you will need to use a carbine length spring + a carbine weight buffer. I would recommend the H2 weight buffer. It is a little heavier than the stock carbine buffer and help reduce wear and tear on your rifle.

You can also use M-16 bolt carrier groups in your rifle with zero problems. They have a little more mass than the semi-auto carriers and help reduce the cyclic rate of your rifle and increase reliability.

As far as I know there are two lengths of buffers carbine and rifle. I'm pretty sure that's even more important then M-16 vs AR-15 issue.

I would use the AR-15 buffer and buffer spring. The bolt carrier group of a M16 is not the same as the AR-15, there are several differences. first the M16 bolt group is longer and has more details and the AR-15 is shorter and less complex. If you are building an AR-15, I would recommend that you only use AR-15 parts only. From my own experience.