Hey everyone!
Bumping this up as I've made up my mind on the engine. Over the weekend I test drove an E85 2006 Z4 30si after someone reached out. The E85 cockpit does feel a bit more tight on my 6'4" body and legs, but I made it work. The key was that sweet 3.0L engine - Wow!!!! No doubt in my mind that 3.0L is the engine for me. Now the key is to find the right car!
Autotrader listings do seem to quite a bit higher than I'd expect -£10-12k for cars sometimes without even the extras I'd hope for (mainly the heated seats and parking sensors - not essential but hoped for). I suspect it'll be sacrificing somewhere on my wish list to get myself in one - either accept higher mileage, or bare-bones spec, or one with lots of extra work to do (suspension refresh, tires/wheels, etc).
Wish me luck on the search, and kind thank you to those who have sent me options to check out so far!