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.

1. Definitely.

2. One way to alleviate the slow research with the all-factories strategy is to get one-way research treaties with the AIs, minor and major.  This will be quite a significant increase, and having them be one-way gives you the flexibility to declare war on them whenever you want, without any diplomatic penalty for brealking a treaty.

3. Hard to say, it depends on too many other variables.

4. Try out #2 and als tech-trading, and if that doesnt work, switching over is probably a good idea if you want to catch up in tech.

Sentient species taste better...

Iztok... welcome back   

It's Good to see you.

You're right in buying as many planets as you can.  They'll increase your strength as they populate.  Make sure you build your military rating up while your going along so the enemy doesn't want to attack you.  The Spin Control Center and a few cargo hulls filled with weapons will do the trick.

The old trick of placing spies on their farms is a good one at any time during the game.  Once the pop dies off, remove the spy for reuse elsewhere.  You can do a lot of economic damage without them raising an eyebrow.

As Iztok said, invade enmasse when you can.  You can build up the fleet and leave them at planets close by your targets.  Launch them all toward the target planets so they reach their destination relatively close together in time.  Don't start the invasion until they're all in position. 

Invade all targets in the same turn.  Don't give the enemy time to respond.  The goal is to completely wipe them out in one turn.  If that's not possible, you can cripple them so that each of their counter attacks become weaker and weaker as they can't rebuild their military.

Also, with the all labs strategy, you'll outperform the AI with your military buildup, even if the tech is low.  Overwhelming them in numbers works better than out teching them, anyway.