He'd be offered 35-40k as a trade in... Those kms kill any chance of a good resale. Go jump on e90post Australian section... There's a quick discussion happening in the for sale sub-forum about current 335i prices. 08 m sport with lowish kms should sell for 55-60k. How much is your m3 worth? If it's anything more than 45k i'd be asking for money your way.
As for comparisons... A tune and some simple bolt-on mods make it considerably quicker than an m3 in a straight line... But driving enjoyment is nowhere near the same. Tuning a high km n54 could lead you to replacing spark plugs, injectors, fuel pump etc relatively soon... But there are cases where 100k+ km engines are still going strong on original consumables.