Purchase Dutch

Can't you just make the purchase through your professional account? The only difference with subject to VAT businesses is that your supplier has to charge 21% VAT and you cannot use intra-Community invoicing (which would result in VAT reverse charge = "0% VAT").
So in your situation, this is a normal business purchase where you paid 21% on the services and/or products.

And then you do indeed end up with a special vAT declaration. You will have to settle the vAT via such a special vAT declaration, because it has been transferred to you. That means you have to do something about it and therefore -> special vAT declaration.

Good, that's the theory :)

Fortunately, you can easily solve this by simply ordering as a private person. Then that foreign company will simply charge Dutch vAT. No vAT will be transferred and you don't have to do anything special.You can still include it in your expenses, by the way. Your company number will not be stated on it, but that doesn't really matter. Just stop that invoice with Dutch vAT in your expenses. Enter the full amount, including the foreign vAT, as an expense.

And in terms of payments, you can simply do that from your business account. Although it doesn't make a total difference where you would pay from. Private bank account, business bank account, it makes no difference to your accounting or taxation.How can the foreign company reverse charge the vAT (intra-Community invoicing) if they cannot include the customer's VAT number on their invoice? The tax authorities will not accept this (at least not in the Netherlands).
The company in the Netherlands is not required to reverse charge VAT if there is no vAT number. In that case, VAT will be charged on the order. This can be done using the details of the Belgian customer who is not subject to VAT.