I ended up going with classic RSTP to get ut up and running, although depending on how long this network is going to be needed, we might "upgrade" it with MSTP.
It is strange that the ES3400 seems to be a "low-end" product, if you look at the feature set and in comparison with smaller brother ex2300 or big-brother ex 4300.
Thanks for all input!
Best regards,
Johan Christensson