For the GL1500 and GL1800 most sidecars manifactures make a frame that is longer as these bike are longer then most other bikes as such not all sidecars will fit this bike. The sidecar also needs to be strong and a bit heavy, I have put about 200,000 miles on Goldwings with sidecars and have owned two GL1500's with sidecars. Our Kenna sidecar is our least expensive sidecar that will work with your bike, it is pleanty strong however it is a bit to light for your bike so ballist would be needed. The EZS sidecars we offer are the finest sidecars made any where in the world and with the EZS you can get automotive wheel and tire packages as well as a new front end for the bike. The reason for a new front end is to reduce trail on the front for lighter steering. With EZS they go with a new front end to also allow for the wide automotive front wheel and tire. On the Kenna I would go with triple tree's to change the angle of the forks for lighter steering. We also offer these. Should you be looking at used sidecars feel free to contact us before you buy as some sidecars by the time you make them safe are not going to be such a good deal, other sidecars can not be made safe ever for your bike. We can also help with mounting hardware for your bike as you will need proper bike specific mounting hardware not "Universal"
Feel free to contact me with any specific questions you may have.