For a former build, I chose a Holset turbocharger. These days, Holsets are a little more proven and people know which units to use over others. However, several years ago much people in the european car scene were just getting into them. Initially, the first benefit of chosing a Holset turbocharger is that they are CHEAP. How cheap? Well, browsing eBay you can find units for as little as $100 used or $450 NOS (New Old Stock). In my case, I had chosen a Holset H1E which I had found as NOS for about $400. Because my budget was around $2500 - $3000 (total), I felt that this would give the best bang for the buck. Avoid eBay china brand knock-off turbos AT ALL COST. Holset is cheap but typically genuine. I have not seen people knocking off Holsets (yet). However, if you search eBay and see things like a T70 or T61 for $250 shipped brand new... that's your first tip-off. Further, if you see no markings on the compressor housings or tags anywhere on the unit that's your second tip-off. Some people talk about their builds and will say "Well I picked up a China turbo just for now" - no! If that unit is placed on your engine, forget "just for now" because you will be saving up to rebuild the entire engine when the compressor wheel shatters and your engine ingests aluminum shards. Please guys - forget the China turbos!
That said, back to the Holset. The units you want to use on typical BMW engines ranging from 2.5 to 3.2L of diplacement are the HX35 series. I chose the H1E because it was new and cheap. The difference in the HX vs H1 is about 7 years of technology. For instance, the H1E has a 3" inlet (really more like 55mm where teh compressor inducer is!) while the HX35 support 4" inlets and even those which are 3" are truly 3" with anti-surge porting. The compressor housings on the HX series are also a little bigger A/R. I believe my H1E had a 2.0" outlet while HX35s will have 2.25 - 2.5". Aside from the HX35, there are HX40, HX50/55, and HX60+. I would recommend not going above an HX40 and really, I'd suggest if you want that higher flow then build a hybrid HX35/HX40. The H1E is similar to the HX40 in general size, but lacks the more modern compressor housing, wheel, and turbine section. It is possible to match an HX35 turbine section to an HX40 compressor section and have basically a "T3/T4" turbo. Once again, you are talking about modifying units so you may need to balance them and or have a performance shop perform the work, but the results is generally good.
Oh - also, when talking about turbocharger, most people will talk about housing sizes relative to A/R. For instance a turbocharger might have a .81 A/R or a .68 A/R on the turbine side. These numbers basically talk about the volume and general restriction of the turbine housing. With less volume you get faster spool, but more exhaust restriction so if you're looking for maximum spool on a 2.5L M50TU then a .58 or .62 or something might work out, but realize in a T3 that might mean only 300 - 350 rwhp support. Holsets do not follow this convention - they use cm^2. For instance there are 8 cm^2, 10 cm^2, 12, 16, 20, and 22. There are more, but those are the ones I've seen. My holset is HUGE. I think it was the 18 or 20 cm^2 T3 unit. That means the turbine side is VERY large, while the H1E compressor housing is not that high of an A/R itself. This is because these units are designed for diesel engines which have totally different VE (volumetric efficiency) qualities. A 7.3L diesel engine displaces a lot of exhaust but since the operating RPM range is so narrow, the compressor housing is sized to provide a relatively linear flow. Most diesels work on a pressure of around 10 - 20 PSI. So, running one of these larger T3 turbos at 7 - 10 psi won't yield much on a gasoline motor since the flow is not typically adequate for what our 24v engines want. Once street tuned, my car running the H1E was "ok". It pulled well until about 6,000 RPM where it felt flat. It was probably due to both my timing map and this turbocharger. The turbo was able to keep the pressure up but the flow just didn't increase. The turbine section was large, resulting in lag but free-flowing, and the compressor side was small resulting in a lack of torque rush - it felt fast, the car had easily 300 rwhp, but it just didn't pull like a train.
Here are some photos of my Holset - you can see just how large it was on the turbine, but how small it was on the compressor side.
