Here's a quick update on this thread. To make a long story short, Amsterdam is a difficult place to shop for a good bikepacking bike on a budget. This is a quick list of some of the bike shops I visited:
De Vakantiefietser
Kaptein Tweewielers
Cor Mantel 2wielers
Beterfietsen
Fietspiraat
Tromm Tweewielers
Lohman Fietsspecialisten
Giant Store (Hans Struijk Fietsen)
Among these, De Vakantiefietser was the most well equipped for bikepacking bikes and equipment. However, they do not offer any second hand bikes, and their cheapest bikes start at around 2000 euros (including a Salsa Fargo for 1899 euros). They carry all kinds of camping equipment, and even stock Revelate bikepacking bags. However, the Revelate frame bag I wished to purchase cost 230 euros, which I found hard to accept given the bags sell for 160 USD on Revelate's website.
Tromm Tweewielers had a large selection of new and used bikes with a variety of prices, and seemed to be the most friendly and professional bike shop I visited.
In the end, I ended up buying the Giant Toughroad SLR 2 new from the Giant store. It was the only bike I could find under 1000 euros with fully rigid frame/fork, which is what I wanted for my long distance on/off road travel.
I found some decent looking options on marktplaats, and looked at one bike from there, but many of the best looking bikes posted were located outside Amsterdam and would have taken too much time for visits.
Thanks for everyone's help and advice!