Gilbo, glad to hear things are moving forward for you, I wouldnt recommend a second hand heat exchanger, bite the bullet get the new one and at least you will know its new and will provide hours and hours of service. Just remember what B.O.A.T stands for "Break out another thousand" Sometimes you just gotta laugh or else you'll cry
Keep us informed and hope to see you on the water sooner rather than later
Cheers and beers
Scott