This might be little difficult to achieve as I am not aware of any such host. However, you can consider getting shared hosting accounts for your clients and you can check with your host if they allow custom private nameservers with shared account (at additional charge).
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Most web-hosts are pay as you go - they have bigger plans you can upgrade to (with the click of a button). However, there are infrastructure and support costs at each level, which are included in the pricing. The price that you are paying for each GB of Diskspace/BW is probably a fraction of the total cost in that pricing level.
I am sure plenty of hosting companies already do this. Start of on a lower package and upgrade as you need to go on!
What is your budget to start off with? as your needing windows I would recommend looking $20+ month for a quality service.
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