Log books
If you can go back as far as 1989 with the logs you should not have too much trouble with a reputable CAMO but as usual the better the records are the less trouble you will have with paperwork.
The real peoblem is the value of the aircraft, people have an attitude at the moment that it is a buyers market and expect to pay silly prices. The problem is there are a lot of old dogs that have high time engines, peeling paint, avionics that marconi fitted and smell like there is some sort of pond life in the upholstery. Just add up the cost of putting these things right and you will see that you are likely to spend IRO twice the buying price. However most people would think you mad for buying a "sorted" aircraft for twice the price of an old dog! and what is more the market expects to spend just over the "old dog" price for a sorted aircraft.