The original 128k Mac is very valuable, they're worth hundreds even in average condition. This one sold for almost $3000.
The sizzling sound may be arcing due to a failing high voltage (flyback) transformer. Charge attracts dust, which tends to be slightly conductive, which causes current to leak around the flyback and accelerate its demise. Simply cleaning all the black dust off the conductors may fix it, but when the internal winding insulation degrades it will eventually fail. Fortunately parts are readily available and not terribly expensive. When the flyback transformer fails it's easiest to simply replace the entire "analog / power" board, but the transformers are available too. CRTs themselves last practically forever - burn-in is the issue with them.
Classic Macs are easy to fix, but don't crack the case unless you are familiar with electronics and especially the high voltages used for CRTs. Apple used bleeder resistors but if it failed open you'd have no way of knowing.
I have not had a single problem with Imagewriter printers. No rubber degradation whatsoever. Vacuum out the dust once in a while and drop a very tiny bit of light machine oil on the carriage slide if it becomes dry. If it becomes dry linear scratches will appear. Avoid using alcohol on the platen. The print heads themselves will eventually wear out but they're easily replaced too. Replace the ribbons before they become too worn or you will accelerate the print head's demise also.
I routinely use a 24 year old Imagewriter II with my OS X Lion iMac. It's never needed any attention. They're bulletproof.