Is it good to strip paint on an antique buffet to restain it

that would be the only way to get rid of the layer of paint/s. Then sand very well and wash down and wipe with tack cloth. Then stain it with your choice of stain. A gel stain will cover imperfections well follow up with polyurethane

If you are restoring it, yes. If you are redoing it to match decor, or repurposing it, a light sanding should be sufficient

I would check the value of the piece before doing any work on it. Sometimes, changing the finish on an antique piece will substantially devalue it. If not, then sand and either paint or stain as you like

First check if it's veneer. Veneer CANNOT be sanded. Sanding veneer will ruin the piece.