The M45 on the Mini is too small for a 4.2, but it works well on 1.6 or smaller engines. It would strangle a 4.2 though.
The M62 from the early Merc 230 Kompressor is about right for a 1.8 to 2.0. I had two of them mocked up on a 3.6 Audi V8 many years ago (one feeding each side) but the belt arrangement was crazy
The M112 from Jags is the right size for the 4.2. If that could be mounted in the V with a charge cooler on top of it, similar to the Jag setup then that would be quite effective. It would be quite a lot of fabrication obviously but entirely feasible.
PES actually did a kit for the 4.2 many years ago using a standard Eaton M112, and it included a CNC'd manifold and charge cooler system, which is what was up on Audiworld a while back. I didn't have 5K spare though
If you know someone with a CNC and / or tig welder then its probably on a par cost-wise with transplanting a B7 RS4 lump. 3D printing prototype manifolds and brackets is entirely possible too, so you can mock everything up before cutting in to a few hundred £s worth of billet aluminium.
I did look at using the 3.0T blower for some light boost as it has chargecoolers built in, but being an FSI engine, the chargecoolers are right where our port injectors go so that's not going to work.
A screw charger from one of the big AMG Mercs would be pretty cool too, but those are very expensive. The M112 can be easily found on ebay for £400.