They are ok, very gutless and underpowered in stock trim. They can be tuned, but not BMW's finest motor ever and often problematic in the p38a.
They are also more of a road going engine, not really suited to off road use. An old school Tdi is a much better power plant for off road use and a Td5 is just a far superior engine to the BMW unit.
There is a company in the UK that does a plug and play conversion kit, so you can drop a 3.0 BMW 6 cylinder diesel in a p38a (this engine used in the L322 Range Rover's). It's not a brilliant engine either, but much better than the old 2.5
However, if it's a play thing, a Rover V8 is likely better and cheaper than any of these options.