I have a 1976 Helms 25 pop-up. I'm glad to say I have no issues with it. I don't have the canvas for it, but its on my wish list.
Can't help you with the repair estimate. I had a soft deck in the same area between the small hatch and the raised deck. It was caused by leaky bolts on the small hatch hinges. I repaired the deck myself.
Good luck on the purchase.
Rehoboth Bay/Indian River, DE
S/Y KJ, Helms 25