There are no direct buses from Sete Rios to Estoril.
Your best bet is to get a Metro from Sete Rios to Baixa Chiado Metro Station, Change there to the Green(Verde) line to Cais de Sodre. At Cais de Sodre you can get a suburban train every 15 to 20 minutes to Estoril. Takes about 30 minutes or so.
