So I got this Rabbit Turbodiesel-

[RabbitJockey]

you can't just swap turbine housings because the mercedes turbine wheel has a larger inducer.  also if you're at high altitude i would highly recommend just rebuilding your current turbo or going to a smaller one like a k14, or k03.   the mercedes t3 is a lot bigger and will be really inefficient at low rpms, especially at higher altitudes, and especially with out an intercooler.  the mercedes t3 is a 45 trim, i have a 50 trim t3 on my rabbit with a 1.9l engine and even with more displacement it has trouble making power before 2500-3000rpms(depending on outside temperatures)  because the compressor is on the surge line at those rpms.  that is with no intercooler. 

the mercedes turbo will work just keep in mind anything you can do to increase flow and efficiency will help a lot since you are putting it on a very low displacement engine that can not rev very high to make up for the lack of displacement

[libbydiesel]

I agree with RabbitJockey.  If you aren't intercooled forget it.  If you are intercooled and don't plan on running more than 25 psi or so, forget it.  If you are intercooled and planning on running pressures that high, also forget it and buy something more suitable, lol.  Running a Merz turbo from a 3.0L engine on a 1.6L engine is going to be an absurdly laggy combination.  The OM617's are laggy at altitude to begin with.  Placing that turbo on an engine with ~ half the displacement means that the boost threshold that the laggy 3.0L engine had at altitude (2,500 rpms?) will happen at close to 5,000 rpms on the smaller engine...  No thanks...

[4x4_Welder]

So I should just stay sane on this then?  I was under the impression that the exhaust side wheels were the same size and the difference was primarily in the housings.  I have played mix and match with turbos before, but mostly T4s. 
Where can I get a kit and new compressor wheel? 

[RabbitJockey]

well, if you're going to be insane you really need to do it all the way and use a better intake manifold, intercooler, and a built injection pump or else you really wont get the full benefit of a larger turbo.   Most factory t3 turbos all have the same turbine wheel, but some older cars have the old larger turbine wheel, and the mercedes t3 is one of them.  do you know what year of car it is off of?   it is possible that its a later one they made 50 and 55 trim t3's on late 80s and 90s mercedes diesels, those turbos may have the smaller more standard t3 wheel but they are really uncommon.

i might have a used but good compressor wheel i could sell you i'll have to look around when i get home.  gpop shop sells rebuild kits

[RabbitJockey]

i do have a vw wheel  if you're interested shoot me a pm