A submission from VW forum – The Samba, visit its website for more tips and advice.
For both of those eras of vehicles I would say the top issue is usually rust. Typically on the floors or rockers but often elsewhere as well – wheel wells, undercarriage cross beams, and so on. Mechanical issues are usually fairly simple and most parts are available brand new from major VW parts places. In my own experience, most used buses need brake work.