YB's these days can easily go to 500bhp, and it will cost less getting this from the stage it's at now than fitting any equaly powefull bmw engine.
the e39 M5 engine has it issues fuzzy, i'd think carefuly a bout what you want from the car in terms of performance. you know the limmits of a YB, maybe it might be worth investing into your YB, you have plenty of space in the engine bay to have the big rads and intercoolers to keep it in check to run 500bhp and you are more that half way there already and if you choose to sell it further down the line you have more chance of getting your money back, cossi lums still go for stron money and its an it has wider appeal to any potential buyer
in all i recon you will need to account £5000 in labour and fabrication and some custom parts to do i right, at the cost of that the engine a box etc. there is lots of trial an error, and even it being a long term project which no mechanic can charge an hourly rate for it eventually tots up
oh wiring will also be an issue, it cause agrro on an s50 conversion so an s62 will be a nighmare unless oyu are brianmoore
have a word with glen as he's done in once. i knw some one who may be tempted but its not going to be cheap either way, this is a £10,000 swop minumim.