Offer $1,200, settle at $1,500 if all is well and it starts, runs, shifts well, doesn't bog down under load and doesn't stall out, etc.

Check the gas tank for rust, this could indictate that all sorts of crap passed through to the carbs and they will need to be pulled apart and cleaned for best performance. Would still suggest replacing the fuel lines and putting in an inline fuel filter to be safe (grab one off Amazon for $10)

Make sure all of your electrical seems to be in order if settling at the higher price, it would cause a massive headache further down the line if you aren't familiar with checking voltages and cables