Insteresting pheneomena. Very cool at high speeds and very hot at idle. I wonder if you have a temp. sensor / guage problem.
Your rad probably has around 2-4 galls of water I would assume so you did not get it all out (unless you were running very low and you did not know it). When you replenish the liquids, put in the recommended 50-50 mix (water / antifreeze). Also, when you are running the car from a cold start, maybe touch both the coolant supply and return hoses and see if they are heating at the same time. This may inform you that you have a clogged radiator.
I doubt that is is yout thermostat after just a couple of years, but you never know. It would not hurt to replace that if all else fails.
The $800 seems stiff for that much work, but you may be talking about Canadien and not $US, in that case, fairly reasonable.
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)