N52 vs N52K - minor changes:
Different engine management (MSV70 vs MSV80)
Different MAF sensor (analog vs digital)
Different throttle body (uses magnetic sensors instead of whatever was used before)
Different valve cover (aluminum vs plastic)
Stronger connecting rods on the N52K
Larger exhaust valve stems on the N52K (these were actually introduced mid production on the regular N52)
Revised electric water pump (no idea what the difference is)
Lighter cam shafts on the N52K (same specs otherwise).

Power output is the same. You'll see some people saying 255hp vs 26xhp and even some quotes of 272hp. The 255 vs 260 are the same engine with the same tunes. Some chassis had somewhat more efficient exhausts allowing a few horsepower to be squeezed out. The 27x hp quotes are in reference to european spec direct injection models.

In the US, only the X3 and X5 came with the 3-stage manifold version of the N52K. But it is trivial to retrofit this to 128/328/528s.

N52K vs N51
Extra catalytic converter in midpipe
Secondary air pump (N52/N52K didn't have one)
Modified combustion chamber geometry
Lower compression
Stainless steel fuel lines
Radiator with ozone fixing coating
Activated carbon filter for evaporative emissions
Purge system pipes made from "optimized" plastic (whatever that means).
Comes factory with 3-stage intake manifold (and thus cannot be easily tuned to make the full 255hp since it already needs the DISA manifold to make its 230).

 

The N51 sounds like a sulev version, so you probably want to avoid that one. A radiator with ozone fixing coating sounds pricey, not to mention the reduced power output.

 

My bmw vin deconder shows the N52 at 258 hp and the N52K at 234 hp ...I guess I'd like stronger connecting rods, etc, though that might add weight and thus the hp loss (in part)?

I'm also revisiting my decision to stay away from turbos. A CPO car would be worry-free for some years, at least...and 335s are out there...how many with manuals, I don't know.

Am I being old and purist for no reason in my anti-turbo stance? I know more expensive when they go wrong...but also know they want the right oil at the right time. I worry they don't have longevity, though.

Is it time to move on to an f30?

 

You're looking at 330s vs 328s. The engines were sold in 3 power variants: Low power, Medium Power, High Power.

1. Low power (325i) 215hp
2. Medium power (328i) 230hp
3. High Power (330i) 255hp

The high power variant has a 3-stage intake manifold. The low and medium power variants do not.
The medium power version only ever existed as an N52K.
The low and high power versions were available in both forms, but in the United States only the X3 and X5 3.0si were sold with High Power N52K, and only the Canadian X3 3.0i got the low power version of the N52K.

Asides for the intake manifold, the engines were identical. Same cranks, same cams, same pistons, etc.

 

The early N52 engines have a CCV system similar to the M54 and M52TU engines, except it has a built-in electric heater - hence the parts are way more expensive...
The N52K has a much simpler and better designed valve cover-integrated CCV, like in the M56 engines.

The E90 SULEV BMWs don't have the fuel pump/gas tank issue that pretty much condemns any post-150K mile E46 SULEV car that needs replacing the fuel pump. The tank has service lids and the pump is a separate part.
They still have SULEV-specific parts, i.e. radiator, airbox, catalytic convertors, some of which are substantially more expensive than the non-SULEV counterparts...

 

Dmax, I don't know if you've test driven an N52.
I have the low power versions in my Z4 and 06 325i and I swear they pull harder than my ZHP. The torque seems to hit at a much lower RPM and just carries to the redline. My ZHP has been sitting in the garage for the past two years. The SULEV E90s have a 15 year warranty on many components, I just never wanted to deal with an N51.

 

After all that, I went b58 in a 2016 340i xD 6 spd...did not like it...neither did my BMW angel--it was totaled by a deer.

I am now enjoying my 2016 M235i 6-spd and my N55. 320 hp, 330 lb-ft