Do It Yourself!
Your looking at a 22 hour plus job and at shop labor prices of today well you do the math. Along the the cost of the parts to be installed.
You mentioned the duovalve also needs replacement. But you made no mention of the Expansion Valve which is connected to the Evaporator and the Dehydrator that also should be replaced. And then there is what ever else pops up!
I just changed the Evaporator, Expansion Valve, and Dehydrator for less then $400.00 on my 420 SEL and it is a doable project for a DIY type of individual. I would tell them all too... well, and do the job yourself and pockect that extra monies you got in your deal.
Just lable everthing real good and place bolts and nuts and what everelse in lunch bags and mark them as well this will help you in putting it all back together.
Or if you don't think your up to it then pay the man and be cool for the remainder of the season.
Hope This Helps!
1987 420 SEL - Like a fine wine only better with age