We bought a 15 year old Stohlquist here on the 'buzz for $200 for my lady on the Grand. It wasn't perfect, but it kept her pretty dry, even swimming in the creeks. I have since washed it with McNett's waterproof/breathable gear wash and revitalized the Durable Water Repellant with a Nikwax product. I expect like-new performance after replacing the gaskets (with socks for the feet). It won't last as long as a new suit, but for the price - and for the right weekend warrior, I think it will last several years.
Whether it's Gore, eVent, Tropos, whatever, the membrane will be waterproof if it is in tact (not cracked or delaminating) and clean. When it takes on oil it takes on water. The Durable Water Repellent keeps the membrane from having to work by adding some beading capability to the face of the garment. I prefer wash-in to spray-on treatments, but for this old dry suit you might try both.
If it fits and the fabric is in good shape, buy it. You can always fix small leaks and learn about replacing gaskets.