Hi Handke04,
Welcome, and sorry for your loss. Perhaps you are a lawyer and know the probate laws and rules for your state. I handled a similar probate, and even though I felt well qualified to give an opinion, the probate judge "suggested" to me in rather direct manner to have it officially valued by an appraiser. I knew more than the appraiser, but I did as suggested. If I had it to do over, I would have first had it appraised by a stamp dealer, because the real value of the collection did not pay for the court approved appraiser.
If you could post photos of the US section up to 1900, I think we could help as to whether it is a "novice", "intermediate" , or "advanced" collector, as most of the value of US would be in that era, unless there are three zeppelins in the air mail section.
And prepare yourself that the actual value may be very much less than most non collectors imagine when stamps are 100 yrs old or more. But it is impossible to tell without visual images of the collection. We are a helping lot, and before anyone jumps me. Yes, sometimes the international sections are more valuable

When I used the American Philatelic Society directory of dealers, I received one hit for a dealer in MO that did appraisals.
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Kenton, Joe, Stamps
(APS # 8094-065512-D) Mr. Joe Kenton
P. O. Box 480456,
Kansas City , MO 64148-0456
Phone: (816) 943-6626
Fax: (816) 943-6626
E-mail: apsjoe2@aol.com
Website:
Business Specialty: U.S. Collections-Worldwide Collections-Appraisals-Buyer
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I know nothing about this gentleman. Perhaps some other members know something of him.
Jim