Euro m3 head will not bolt up to an m50/2
an American s50/2 head will(m50 non vanos,m50tu and m52 single vanos heads are all interchangeable)
The oil filter problem can be solved by cutting and welding it shorter and using a different filter. Can't remember what filter off my head
Or you can use a euro m3 oil filter. Only thing that needs adapting is an oil line for the vanos. Plus you get the benefit of an oil cooler.
If your going to all this effort you might as well fab up some standoff injectors while you are at it.
Tuning will be the most expensive part of itb's and as said all ready a lot of work for not a lot of gain.
In saying that i'd love to buy some and try my darndest to fit them. How much do they usually go for in the uk?