I've got a 94 build 4.6HSE which is so far averaging 60k between engines. Reading the paperwork, 2 failed due to poor maintenance from the dealership. Also the standard RV8 is an accident waiting to happen, especially on LPG. It's not perfect, but I've probably spent about 12k over the last 6 years on putting things right, and of that 12k, 5k has just been spent on putting in a Turner Engineering top hat linered block with Gas Flowed heads and Mark Adams performance injectors and remap. It's already got the sports cats and stainless system. I was questioned on why I never have the radio on recently. It's not broken (far from it in fact). I just have to have the window down to listen to the engine purring away. I love the V8. Don't care about fuel cost or cost of running because I enjoy it too much.
I've driven a few DSE's and frankly they are very poor. Generally they keep going but if not maintained the head will fail. Some are tractor like. I'm sure BMW thought their M51 engine was a good idea at the time, and maybe it is in a car, but not a full time 4x4 weighing in at 2 tonnes. Mapping them is fine but you need to keep a close eye on things. You also need to remember that owners are using parts from BMW breaker yards so what you think you are getting, may not be what you are actually getting.
So thinking LPG'd or diesel, they aren't cheap to own. However, it is a buyers market so you can find a bargain IF you take your time. I know one for sale at the moment which is on ebay at a reasonable price. The owner is an enthusiast and being a mechanic he makes sure all the basic work is done to a high standard.