Sanding stairs is a tough job. Most of the work is done to detail and much of it is manual work - simply hand and abrasives. I would not encourage restoration of painted already banisters if they have ornaments and shaping on them. If the design of the banisters is more simple without many "hideways" and difficult to reach for the tools and abrasives places, than stripping paint completely of the staircase is an option which you can consider and expect good results.
If you are to seal the stairs with clear product, even coloured and you wonder wether to choose lacquer and varnish, I can make a point here which you can think about. Wooden staircase can take hard traffic on stapes and less use on the banisters. After you see how much hard work is thrown into staircase restoration you may also consider using hardwax-oil for banisters and lacquer for the steps. It will be much easier to restore one day again when this is necessary.
Both products can be found in matt and satin. Osmo hardwax-oil is one of the products offering 2 sheens.