The simple answer is to make sure you pay particular attention to the drains before you purchase. In a lot of cases the old drains may actual be in a reasonable functional condition and no replacement would be necessary in the short to medium term.
Fairly often in older properties there would be very limited or indeed no access to drains outside the property. If there is access the drain covers should be lifted to allow for examination. Cracked pipes or failed connected including root ingress at the drains will give you a good indication of what to expect in the sections of the drains that you cannot see.
Also, settlement or cracking in paths over the drains will indicate soil movement that will almost certainly result in cracking in the drain pipes. The presence of both large or small shrubs or trees less than the tree height to the drain will in most cases allow root penetrations into the drain pipes.
To know for certain what is happening with the drains concealed under the house, footpaths and grounds of the property can only be fully determined by using a cctv camera. Depending on the number and extent of the under ground pipes expect to pay in the region of €200-300 + VAT for a cctv inspection. In most cases an experienced surveyor will be able to give you a good idea about the expected condition of the drains based on the sections of drains he can see, settlement or movement in the paths over the drains and cracking movement in walls adjacent to the under ground drains.
The main concerns is not only the cost of replacing the old drain pipe/s but the more expensive damage that can be caused to foundations if a drain leaks in close proximity to the foundation of an old period property.