The Pentax FA 28-70mm f4 is a very good lens. I don't know the prices in Europe; in the US it sells for $80-150, depending on condition and the place you get it from. So, the price might be right. But the lens is known for being prone to lens separation--so check that aspect, to make sure your copy isn't affected. Read some reviews here (which also say more about separation):
SMC Pentax-FA 28-70mm F4 AL Reviews - Pentax Lens Reviews & Pentax Lens Database
The Sigma is an average lens. I'd advise you to skip it. Not worth 100 Euros.