I’m with Karin – Omron makes great automated BP units. I’ve been using the same one for almost 20 years now. I’m sure the ones today are even better. As long as you follow the directions, you should get accurate readings from it.
I have been using a blood pressure kit from Omron for years. You can buy the larger cuff as an accessory. Omron has several models available and has a very good record for accuracy.