This guide is written to help others that are just getting started in the hobby and writing this guide as have many years dealing in buying and repairing radio equipment.
Don't ever buy any transceiver that has been modified in any way, unless you know what you are getting into! This is especially true of microphone, power, and extended frequency mods. Microphone mods remove the noise canceling feature, and allow every nuance of road noise to be transmitted. Adjusting the final bias and/or drive level is a sure-fire way to shorten final transistors life, and drastically increase the transmitted, third-order IMD. Extended frequency mods are often done citing MARS use, when in reality for most radios it is to allow 11 meter operation. All of these mods can potentially damage the radio and can make the transceiver illegal to operate!
I personally won't consider any transceiver older than 15 years or so, unless I was planning on collecting, or after a particular model. Sooner or later, every transceiver on the market will become obsolete and some have several revisions might have been done to the same exact model over the years. Hidden below the surface, are the numerous internal changes necessitated by discontinued parts, or design flaws. As a result, any given two, identical appearing radios same models, might have different finals, or several change done to various stages. Whether those parts remain available is moot. Best bet if your after a particular model, do the research and find out what changes were done and why and look at the serial number ranges. You also have to take into consideration as radio ages, various components fall out of tolerance and radio will less likely operate within specs and more likely be very expensive to service. This also hold true with new in the box radios that been sitting. Reason why I believe it best to avoid radios that are over 15 years of age especially at high prices, unless you know what you are getting into.