If I understand your question you would like specific values assigned to each resulting raster buffer. This can be done by simply assigning values via a index bracket assignment.
First, create a raster with a polygon to buffer.
p <- spPolygons(rbind(c(-180,-20), c(-140,55), c(10, 0), c(-140,-60), c(-180,-20)))
r <- raster(ncol=100, nrow=100)
r <- rasterize(p, r, fun=sum)
Then we can create buffers and assign values using a bracket assignment "". The "!is.na" within the bracket is a conditional statment that only evaluates values in the raster that are not NA. You can then just print the raster object to get some general information or use summary to display summary statistics on the raster values.
b1 <- buffer(r, width = 1000000)
b1[!is.na(b1)] <- 100
b2 <- buffer(r, width = 2000000)
b2[!is.na(b2)] <- 200
Here are the plotted results.
plot(b2, col="black", xlim=c(-200,100), ylim=c(-95,95), legend = FALSE)
plot(b1, col = "green", add = TRUE, legend = FALSE)
plot(r, col = "red", add = TRUE, legend = FALSE)
The trick to combining the buffers is to turn the background (NA's) to zero, so an overlay will not just return a single buffer, and then set the zero background back to NA. You can then use the "mask" function in raster to clip to additional features (eg., coastline).
b1[is.na(b1)] <- 0
b2[is.na(b2)] <- 0
r.sum <- overlay(b2,b1,fun=sum)
r.sum[r.sum == 0] <- NA
plot(r.sum, xlim=c(-200,100), ylim=c(-95,95), legend = FALSE)