I recently bought an '81 300TD and this thread covers a lot of stuff that may be useful to you:
Help with 300TD...?
Also, look under the DIY "sticky" at the top of this forum. Within are links to another website that include a w123 buying guide. There is also a British w123 club on the web that has a pretty good online buying guide. I don't have time to look for it now otherwise I would post the URL, but I found it on Google and so can you.
This forum is great. Post any specific questions you may have after looking at the car and you'll get lots of help!