How many estimates have you gotten? If the "between $25,000
and $30,000" came from a single roofer, then you should get
at least two more estimates. Don't just look roofers up in
the yellow pages -- talk to homeowners you know who've had
their rooves done and get recommendations.

An estimate of $25k to $30k for a tear-off sounds reasonable
to me, but again, you need to get other estimates.

Was "the estimate from some guy for around $8000" that the
selling agent told you about for a tear-off or putting another
layer on top (which would of course be completely
irresponsible if there are already 3 or 4 layers)? And what
type and quality of shingles did that guy intend to use? If
you don't have details like this about that estimate, then the
estimate is worthless. I suspect it's worthless in any case,
because the agent probably found someone he knew would
low-ball the estimate so that he could quote you a low
estimate for redoing the roof.

I doubt that it will be cost-effective for you to salvage the
old slate. If a new roof was nailed on top of old slate, then
there will be nail holes through most of the slate shingles,
and many of them will be split; all of the split or holed
shingles will not be reusable. Furthermore, until you've
removed all the layers on top of the slate, there's no way of
knowing how much of the slate is missing. It seems likely
that there will be quite a bit missing, since if there weren't
they would have patched it rather than putting on a new
asphalt roof. Another issue is that even if a significant
amount of the slate is reusable, it will be difficult for you
to find new slate to match it for the rest.

I doubt you'll be able to find a roofer willing to do what
you're proposing. If you do, he won't be willing to give you
a reliable estimate until the layers above the slate have been
removed so he can see how much of the slate is reusable.

Slate lasts much longer than asphalt, at least in climates
where its use is not recommended because the weather is too
harsh. It's per year cost averages out significantly lower
than asphalt, and hence it increases the value of your house
and costs you less in the long term.

If you haven't yet closed on the financing for the house, and
you know that you're going to have to replace the roof, you
might want to consider rolling whatever portion you can't
afford now of the cost of slate into your financing, so that
you can go ahead and put on slate.