Need advice on navigating a collection purchase, including T206s

[Four Pitches]

Often reader, seldom poster.

I've purchased several collections over the years, from hundreds of dollars into the thousands. This one is larger so I'm in need of some wisdom!

I've an opportunity to buy an inherited collection from someone whom I've vetted independently and believe to be legit.

Seller is not interested in going through the cards one by one as they have a couple hundred thousand cards and are no more knowledgeable than what an eBay sold listings search will tell. Problem is their asking price is pegged to a handful of ungraded cards, including some nice T206s. Second problem...I've never paid this much for a collection and don't exactly have this kind of money laying around burning a hole in my pocket. On the upside, if the cards are genuine the value would be much more than asking price.

They've sent several pictures of cards that are worth $1-2k each, so I do believe this collection to be valuable. I have no reason to doubt the seller, but I also can't look through so many cards in an hour or two!

Here's where I need help. My strategy - 1) go see the cards (duh). I'll bring comparable T206s and others, along with my loupe for close inspection. I won't have time to look through all the cards so I'll have to choose a few boxes and estimate a value based on what I see. 2) assuming I like what I inspect, make an offer contingent upon authentication of a few of the key ungraded cards.

I need access to the cards for days(!), however, to go through everything. So I'm considering an up-front payment--earnest money, if you will, to get the cards in my hands. And if the raw cards are then successfully authenticated, a larger payment would follow. Am even open to setting this up contractually. I just can't lay out a large sum when I'm unsure about authentication. I have a mortgage, after all

I'm sure some of you have done something similar to help finance a larger purchase. Any advice you can offer on what worked well? What did not? Changes to my strategy?

Additionally...I don't think seller is going to be too interested in letting me play around for extended periods. I'll need to have the authentications done and have him paid fully within a few months.

Any help is appreciated!