I can see why you could tire of a loud fast performing Mini, particularly if it is usually driven relativity sedately. However, there is no need for a modified car to be loud or badly behaved.
Why not go for the best of both worlds, a sensibly modified car.
No Mini has ever not benefited from the fitting of a stage 1 head, this will only improve things, quieter, faster less fuel consumption, a decent exhaust manifold & why not leave the 1 1/2" carbs on, but fit a decent air filter, possibly using a modified standard air box. The standard cam is pretty poor, giving a good tickover, but not really offering much more, fit a 997 Cooper cam, an SW% or a Minispares high torque. None of these will make your engine misbehave, all offer advantages & will make your car nicer than standard.
For example look at a Downton No5 Touring Conversion, this made a Cooper S better in every department;
http://mk1-performance-conversions.co.uk/dtn_45.pdfA car modified in this way was a great improvement, not only in performance but manners too.
Don't get me wrong, I like standard cars, but a well modified car will always be preferable in my eyes.
As far as value is concerned, I honestly don't think there is any difference between a well modified car & a standard one.