I think you will have it easier to find a Getrag 265 in Europe than a 3.27 rear end (specific to the US market). I would guess that finding a 3.46 or a 265 would be the same level of difficulty. Many many BMWs were fitted with Getrag 265s so they should not be that hard to find. Get on some BMW forums. They are very strong transmissions. You would need to get one with the speedo drive. It is better to pay a little bit more for a known low mileage transmission than one that needs some work. Then just replace input, output and shifting shaft seals and you're good to go.
Swapping a rear end is less work than fitting a Getrag 265. However, you would most seemingly need to at least reseal the rear end and replace the center pivot bushing as well as the center boot. This would bring the work involved quite close to the one needed to fit a Getrag 265. Both jobs are doable on your own, and Mark Turnbull has developed a nice adaptation kit for the Getrag.
Both swaps are reversible, and do not require any structural modification. So in terms of value, I guess it will all depend on the buyer. I would personally value a 5 speed trans more than a rear end swap, especially on a 230. The 230 is a very lively and sporty engine and the car will be much better to drive with a 5 speed and a stock rear end ratio than a 4 speed and a different rear end ratio. Of course this is according to my personal taste, so I guess it's a bit subjective.
The five speed, together with electronic ignition, is part of the upgrades that make the cars of that era more enjoyable and more trouble free in todays traffic conditions and make long distance trips a real pleasure, while keeping all the peps on twisty mountain roads.
