... you can't present perfect (or continuous) and past simple within a sentence.
As it stands, this rule is incorrect. In many cases it is acceptable and logical to mix past and present references in consecutive clauses
I lost my keys last week, but now I have found them.
This makes sense: A was true then, but B is true now.
This, however, does not make sense:
He has decided to go hiking, so I went hiking as well.
This sentence amounts to A was true then, because B is true now. The simple past describes a past event, your going hiking, but what the present perfect describes is not a past event, his decision, but a present state which is the result of a past event--his state of having decided. That present state cannot be the cause of the past event. The cause must be either a past event or a past state:
He decided (event) to go hiking, so I went hiking as well or
He had decided (state) to go hiking, so I went hiking as well.
The important thing is not to mix time references illogically.
As for the sentence in your friend's email:
Getting that email was such a pleasant surprise, because I was just thinking how I've been wanting to send you an email
There is no mixture of time references here, because the progressive construction "I have been wanting" marks a state, not an event, which may very reasonably be taken to continue into the present out of a past which is marked (by "just") as immediate. In effect, these pasts inhabit the same time frame as the present.
In any case, the "rules" are very loosely applied in informal discourse; see my discussion here. A casual email, which your friend probably dashed off in excitement, should not be held to the formal literary standards of coherence.
It looks grammatically correct, but it doesn't sound right because people don't use it. "Would you be interested?" is the more common way. In fact, I've only heard non-native English speakers use "Would you have interest."
When I taught EFL years ago, I would often revert back to the "it doesn't sound right excuse" and when pressed more, I would delve into a boring grammar reason.
Delving a little deeper, interest, when used in the active voice, requires an object. It also requires an article in front of interest.
For example: Would you have an interest in this job?
Again, it still doesn't sound right. Would you be interested in this job? still sounds more common.
Best Answer
Both versions are perfectly fine.
In the case of
"you" is an indirect object. It is understood that the subject is not sending "you", but rather sending the email.
Personally the first version,
sounds a little stilted.
In conversational English, you would probably use email as a verb.