I see you're in NJ so I'll assume that you plan to use the boat in salt water. If that's true then the steel trailer is not worth much, especially if the brakes need repair/replacing. If you do buy this and you're going to be using it in salt water, I'd suggest buying a used galvanized or aluminum trailer and not put any money into the steel one.
The asking price is not bad. I have yet to see a Grady, including the one I bought, have a price any where near as low as NADA values, even in today's market.
In my opinion (and we all know what they're worth), if the boat meets your needs and you are comfortable with its condition then buy it, especially if yo're comfortable with the seller. There is no guarantee that spending more money on a different boat will result in fewer problems.
You know the risks with the transom and fuel tank and said you plan on replacing the motor in the future. These considerations apply to all boats, not just the Grady.
Since his price is firm you might try get him to include some extras like the safety equipment (PFD's, flares, etc), docking lines, anchor and rode, etc.
Another thing for you to consider and I learned this the hard way. A trailer that only gets used twice a year is likely to have bearing problems if you try to tow at higher speeds and a longer distance. I suggest you try to get him to replace all the bearings and races as part of the deal.
Gary 02 Overnighter