I have recently adopted the buy-conquored-planets strategy. When i see a planet with about 1 billion or below i trade for it. Its one less planet I will have to get later and it also adds to my influence. Most importantly those planets usually give me a foot hold in an area of the galaxy I wouldn't normally be for some time.
Your specific situation may be different than mine though:
a) I had the Resturant of Eternity and a few influence resoiurces. So i didnt worry about those planets flipping away.
My tech rate was massively above the AIs'. So i Could throw in some techs they didn't need or that couldn't come bachk to hurt me instead of cash.
c) I had all of the extreme envirnoment techs.
d) I had the economy to be able to BUY improvements on all of those planets when i got them.
e) My tax rate was 80%, so even with only 1 billion people and some existing improvemnets I was making a profit.
So to actually answer your question. YES, go ahead and buy those planets. The more you have to more powerful you are and the less the AI is.
edit: i like how that smiley snuck in there.