I owned a used 1983 Islander Bahama 30 for almost 10 years. The Bahama is essentially the same as the Islander 30, but with a less "teaky", i.e. less expensive interior. Actually I preferred the brighter interior of the Bahama, but that is just a matter of preference. I liked my boat. It was powered by a 13hp raw water cooled Volvo MD7. I never had a problem with the engine, but I was sailing in fresh water. Obviously these old loud, vibrating beasts are expensive to repair or replace, so look carefully at that.
The boat has a very shallow bilge and water damage to the cabin sole can be a real project. This was compounded by the fact that the icebox and the shower drained directly into the bilge, with no sump. Not the best design in that regard.
With the deck stepped mast, take a close look for any deck weakness due to compression post issues. I never had any problem with my deck or the glass work, in general. Blistering, though, was a problem on my boat.
For the most part, boats of that era seem to have come with pressure alcohol stoves/ovens. What a PITA
I replaced mine with a propane stove. I believe the standard water tankage on these boats was only 20 gallons, although an additional 20 gallon water tank may have been added as an option.
On the plus side, for a 30'' boat, these boats were great for the type of sailing I was doing. The cockpits are very comfortable, and the boat sailed reasonably well. I think they sail better if they don''t have too large a headsail. I routinely kept a 110 on my furler and felt I could keep the boat on it''s feet better with a smaller headsail. Even in relatively light air, I thought the boat was a decent sailor.
In summary, I think the Islander 30 is a decent boat. A good survey is probably money well spent, as there could be some expensive "issues", but probably none unique to Islander. In general, I think these boats are comparable to Catalina 30s of the same vintage, but I preferred the Islander. These boats were/are very popular where I sail (Lake Texoma).