You're asking for advice but you seem pretty set on getting a 640-1 no matter the advice. There's nothing wrong with wanting a 640-1 if your finances allow it. If they don't the 640 no dash should serve just fine.
I've had a love / hate relationship with our 640-1. It's the first pistol my wife and I bought together. After shooting it the first few times i was ready to sell but my wife was sentimental which led to holding on to it and occasionally breaking it out. With some light customization, experimentation, and a lot of dry fire it has, over time, become a favored shooter. Changes include Taurus grips and painted front sights. Dry fire has smoothed the trigger to custom good. At this point it is old shoe comfortable and can hit the upper A Zone consistently at 25 yards. I wouldn't expect POI to equal POA but with tuning and practice you will know where to hold over, etc. If it's what you really want, go for it...