I had the earlier version (Mk1 non - IS) of that lens and loved it. The AF was fast and accurate and the IQ excellent, even with extenders. However the weight (6.1 Kilos) was a bit daunting to say the least! I sold mine simply because it was not long enough for my needs - lovely lens though.
Unfortunately the lens you are considering is, as you state, no longer supported. These lenses were built to last, but everything has a finite life. This should be reflected in the price you pay as you are taking a bit of a chance - it may work for years (and probably will) but if it falls flat tomorrow then it may not be repairable. You must balance this against the much higher price (and reduced weight) of a newer version.
Personally I would not pay more than a third to a half of what I would pay for an IS version.