Hi
A shock absorber is one of the most expensive parts on a motorcycle.
Wilbers is an expensive brand. YSS is a reasonable option.
How confident are you working on motorcycles.
Old motorcycles can have many issues and it can be expensive paying someone else to do the work.
I have a few ideas, based on my 20 years of ownership experience with a 1984 BMW K100 and a 1992 BMW K75s.
-familiarise yourself with the bike. Download the owners manual and the official BMW service manual. They can be downloaded for free.
-clean all the electrical connectors, especially the connectors under the petrol tank. Use a contact cleaner with an extended nozzle. Spray it into the open end of the connector and make certain you have a rag on the other end of the connector to catch the residue. It is not difficult to remove the petrol tank and it’s easy to disconnect all of the connectors.
-remove all of the fairings. It’s not difficult, especially your bike. It will give you easier access to your bike’s components.
-remove the rear wheel and the final drive. Once you’ve done that, check the driveshaft, in particular the splines.
The splines can be expensive to replace.
- eventually replace all fluids and filters.
-Please check that all the systems, that is, brakes, lights, fuel injection, engine, switches etc are all operational.
-a new shock absorber, which can be expensive should be one of the last purchases. No point purchasing a shock absorber if the rest of your bike is not running.
-Motobins and Motorworks BMW in the UK are a good source of cheap parts. It also depends on the currency exchange rate and postage costs.
-the official BMW dealership is where you will find the hard to get parts, especially the fasterners.
Good luck.