The facts (if you would rather deal with facts rather than fantasy) are that most of the 'parts' on the mk1 and mk2 are the same, or similar. Much of the suspension is interchangeable, the engines are similar in design with similar quality parts, the subframe, wishbones, mounts, all similar. The gear linkage, electric motors, pumps and drive belts are all similar. Just where this improved reliability in the mk2's has mysteriously come from is something i wish the delusional would tell me.
I expect some more c0cky one liners in response to this post, as this hides the fact that the respondents really know very little about how their cars work or how the parts function.
There is no real evidence that either mk1s or mk2s are more reliable. There are shockingly unreliable examples of both and surprisingly reliable examples of both.
I suggest you try a Williams and see if you like it. If the service history is good, the oil looks fresh and the belts new, id say it will probably be as reliable as any mk2. The engine are pretty bullet proof.