Hey Nick,
Welcome from Ct....the 1500's are older but I haven't found any issues getting replacement parts for it. I just got a new seat for my 1500 in March from Partszilla along with a new CB antenna. I have also ordered through Cyclemax in Ohio and Dennis Kirk. The only thing I recommend is to possibly change the timing belts depending on mileage or if they have been changed. These are available through auto parts houses locally. There are a couple of small filters that need changing also in the air supply lines.
There is a good web site on line goldwingdocs.com that has a lot of good info on it, including a list of vendors for parts and how to articles for working on the bike. It's not a trike site but does have a forum that I see trike posts occasionally, but for the basic bike it is great. I had a clutch issue after sitting all winter and followed the how to to flush and bleed the clutch fluid. When I did my timing belts, it even covered removing the plastic so I could work on the engine.