I'm considering looking at 2009+ 8th gens for my next car, but I have some questions regarding performance, feel, reliability, and ease of DIY repair on the 3 V6 engine/transmission options.
As I understand it, pushrods are typically simpler and easier to work on than DOHC motors, and considering that these big V6s are in a FWD engine bay, I'm conserned about work space and reliability of the motors when repairing things myself.
Which motor would you suggest if I wanted one that was gonna last a long time and be the easiest and cheapest to repair without feeling sluggish and wasting tons of gas?
I recommend looking at 2012+ model years with the LFX 3.6L engine and 6T70 transmission. Yes, it's an OHC engine, but it seems to have a very good reputation, as does the 6T70 transaxle.
Here's a list of engines I culled from Wikipedia for the years you mention.
LZ9 3900, 2006–2011
LGD = LZ9 flex fuel, 2009–2011
LZ8 = LZ9 w/DOD, 2007–2008
LZ4 3500, cut-down 3.9, 2006–2009
LZE = LZ4 flex fuel, 2006–2011
LFX 3.6L, 2012–present
These are all 60° V6's, so none of them are particularly tightly squeezed into the engine bay - none of them are fat like the 90° Shortstar (3.5L LX5 DOHC V6) in the related w-body Oldsmobile, Intrigue.
The 3.9 and 3.5 were both cam-in-block push-rod motors. The 3.9 LZ9 was a new design, and the 3.5 LZ4 was a cut-down version of that. These 2 motors replaced the older 3.4 LA1, but they were subsequently replaced by the 3.6L LFX, which was used in the last of the gen8 w-body Impalas and is still in use in the newer gen9 Epsilon2 Impalas.
The only knock I've heard on the 3.6 is changing the rear plugs, but there's a Youtube video that shows a neat trick wherein the guy turns the COPs 90° to get them out - voila! - no need to pull the intake. (Sorry, don't have the link handy.)
My limited experience with the 3.9/3.5 series was the LZ4 in my Pontiac G6. They were known for a coolant leak in the crossover pipe on the passenger end of the motor, but seem to otherwise have a decent reputation. I did the fix on that, and it wasn't too difficult other than dealing with the power steering pump pulley, as I recall.
As for the 3.9, the LZ8 DOD version (displacement on demand = active fuel management = shut-down cylinders) lasted only 2 years. The DOD stuff was a problem in the 8-6-4 Cadillacs 35 years ago, and it's a problem in the more recent V8 LS4 Impalas. I would definitely avoid this.
And welcome aboard!