Re: Steel boats - things to watch for?
3mm not unusual for top sides esp. on a smaller boat.
4 - 5 mm for hull as a minimum.
Steel boats die from the inside. Ignore the outside surface rust get inside with mirrors and good lights and a hammer. An ultrasound examination carried out by a surveyor used to working with steel boats would be a good investment if you have doubts.
I cruised an old steel boat for 7 years and had some patching done when problems appeared. Relatively cheap and I knew that repairs were as strong as the original.
That Tahitiana looks good if you can cope with a slow boat. Several have circumnavigated.