I'm biased obviously, owning a x-flow, and acknowledge straight away that more modern and more powerful engines catch the eye of prospective owners out there....
For simplicity, the x-flow takes some beating, and if you like to wield a spanner, then there's a lot you can do to them maintenance wise without too much effort, and then, there's the soundtrack, that really does take some beating....
X-flows can vary a lot too, bearing in mind its a 40 yr + old engine - basic unfettled examples will be right at the bottom end power wise, and will soon feel slow, whereas a top notch example with all the mod cons and 140+ bhp will deliver smiles per miles imho equivalent to much more expensive set ups approaching 180 bhp....
But they are old technology, so bear that in mind. I would say an absolutely top notch x/flow in anything less than a concourse car & chassis would be 6.5k absolute tops, with lesser examples down to 5k plus.
Don't forget bhp is only on paper too, car set up, brakes, suspension & tyres complete the package and many under-gunned setups on paper are much closer to just bigger power set ups in real driving terms...sitting a few inches off the deck amplifies the experience and legal speeds feel like well north of 100mph in boring tin tops.
Mart.