What would you do????

Spend the 20K, you know the truck, you like the truck (or you wouldn't be asking the question) and it gets good fuel mileage for the job you're doing.

 

Unless you are buying brand new I would keep it at lease you know whats wrong with it and whats been done to it. Getting another used one could cost you more than $20K and still give you a bunch of problems the seller fail to tell you about

 

Definitely fix the existing truck. And don't spare any expense. Paint job I did on my 12 year old truck and the reman engine just before that 2 best things I have ever done. Now I am going through the interior(plastic Freightliner dash is busted up bad). Pay close attention to wiring. Most of the other stuff you can see and replace before it leaves you stranded.