As Mountain Lion (10.8) had been released this summer, there is no easy way to go to Lion only.
In Mountain Lion, iMessage is included and does not need to be downloaded separately.
To find out if your Mac Book could be upgraded to Mountain Lion, you might use the free MacTracker app (available in the Mac App Store).
There you could see which model you have and if Mountaion Lion might run on it.
If this check is positive, just buy Mountain Lion from the Mac App Store.