The mentioned yards by you are well known (in Holland), good quality yards. (I have a 43ft Koopmans centerboard
, contructed originally in 1985 at Aluboat in Hindelopen, top quality contruction)
A well know broker is "van de Valk" in Holland
. There are many others however who I think will do an equal good job. Van de Valk broker is sitruated all over Holland, so they can set up with several offices. My boat was located in the South, van de Valk set me up via one of the office in the North. I am Ducht, live in US now, otherwise would have helped you out.
the website www.botenbank.nlis a good website for boat searching prior getting in Holland. Type in "koopmans" or "van der stadt" in search box "merk / model" , press "zoeken" (search) and you get everything available via Dutch brokers.
Aluminium or steel
, most important thing is only take it when iot was constructed on a good quality yard, as about the yard and how many they have built over the last 10 years etc. In the case the vessel was built on unknown yard, then you should have good survey
done, by someone familiar with metal construction. It really comes down to the details in metal construction. Yards like aluboat go into those details.
success, peter a s