Appropriately sizing a turbo to a specific engine in a specific vehicle is complicated. A turbo that is small will spool very quickly and yet is small enough that it can be a bit restrictive, potentially hurting high-rpm performance. A turbo that is large will have significant lag where no boost at all is developed. When the larger turbo hits its "boost threshold" it will start creating boost and will quickly rise to whatever boost pressure is being regulated by the turbo wastegate. The larger turbo flows much better at higher rpms than a small turbo and so is less of an exhaust restriction and will give better high-rpm performance. The decision of what turbo is best is very subjective. Many people hate turbo lag and so want a smaller turbo. Many people don't mind the lag and prefer the bigger punch when the turbo spools and find the better high-rpm performance desirable. With regard to specifically the vanagon, I have always preferred to have more boost in the lower rpms in order to help get the big box moving. I don't tend to rev it high and so having the boost at lower rpms is important. The other factor to consider with the vanagon installation is whether or not the turbo interferes with any of the engine mounting components and what work is necessary to eliminate that interference. There were basically 5 different turbo-chargers used on the 1.6TD and 1.9TD engines. They were the T3, K24, K14, T2, and K03. The T3 and K24 have the same boost characteristics and are both just plain too large for the 1.6TD in a vanagon. Boost doesn't start until about 3,000-3,500 rpms and you can literally get stuck in 2nd gear on moderate grades and even if you rev it to the limit, you cannot shift fast enough to be above the boost threshold in 3rd and without any boost you cannot accelerate. This effect is most apparent at higher altitudes, but those turbos are irritating even at lower altitudes. The K24 and T3 also both severely interfere with the driver's side mounting bracket. The T2 and K03 are the smallest of the 5 turbos and share the same boost characteristics as each other. Their boost threshold is below idle and so you will see basically instant boost when letting the clutch out. Full boost is developed by 2,000 rpms. They don't ever give the punch of the larger turbos because they are always developing some boost. They do restrict the exhaust more than the larger turbos and so comparatively hurt high-rpm power some.The last option is the K14. It is middle-of-the-road as far as size is concerned. It is a much better turbo than the K24 and T3. It flows as well as the larger ones at higher rpms and yet spools at least 1,000 rpms sooner. There is some lag, but you definitely feel the boost kick in more than with the k03. The K14 flows better at high-rpms than the K03. The K14 is a significant interference with the driver's side mounting bracket.
You can, and in fact should, clock (rotate) the center section of the turbo so that when it is installed in the vehicle, the oil inlet is at 12:00 and the outlet @ 6:00. That will alleviate the manifold interference. Regardless, to clock the turbo properly when installed, you will need to remove the oil return pipe. Best approach, IMO, is to use a banjo bolt and fitting in it's place and run a custom oil line off of the banjo fitting.Here are the best pictures I have of the modded bracket:
