The fuel consumption and cruise speed seem realistic enough. They are economical engines. I suggest looking into parts availability for Tempest marinization parts, you should be ok for the Ford parts. I have Lehmans, but it`s the same base engine.
My Owners Manual shows the LWL as 32ft 10",for the fibreglass version. No info is given for wooden versions.
Do you mean 3-4M swells? If so, it should cope if they are not breaking, but if you mean wind waves....NO. I experienced 2 M waves forward of beam on,didn`t enjoy it at all. They are a good coastal cruiser, but choose your days.
Are you retaining the shipwright who maintains the boat for the seller as your surveyor? If so,an unusual arrangement.
I keep an eye on IG36s on the market, I`ve not seen this one. There was a wooden one sold on Lake Macquarie recently,but I think it had the aft cabin.
The usual problem with IGs like this is deck issues top and bottom and window issues(rot), and resulting water ingress. Rusty steel fuel tanks are possible, esp the tops with water ingress,but mine and (I think) 2 other Australian early 1980s 36s on TF still have original tanks.
With a wooden boat the hull is an obvious concern, but same applies to the fiberglass versions prone to osmosis.
Feel free to ask anything further. Other members may have other thoughts.