I have a year older of the same model - with a "few" more hours on it
These tractors new sold for about that price - so you are not getting a "deal" as of yet.
However - with the thumb and forks - those are bonuses.
The fact that getting anything new is like a finding a needle in the proverbial haystack you may just want to have the cash in hand and start counting it off to the old owner. To me - the price is a bit high - I would offer 17K and see how that works and then creep up. The machine is worth every penny of the 18.9 K but if used machine were not so high now you may have success with a lower price - but used are almost as expensive as new!
WITH 90 hours - has it had a HST filter replaced? I would really "demand" the maintenance paperwork if he has it. With 90 hours that machine is barely broken in, still a newbie!
AS for gaskets - all of those are fine!
IF/when you buy - first thing ..... get a set of rear spacers, 2" would be perfect, especially since you do not have a MMM!
Second thing (maybe more important than that first thing) - get an under carriage shield - if you ever break those fan blades up under - you will wish you paid the $300 bucks for the underarmor.
Another purchase to dig with would be a toothbar - you may have to wait a bit to get one, but it is worth the money and the wait.
The machine you are looking at is a total package - it can move mountains ..... just one little bite at a time. It is NOT bullet proof - but damn near!