Hi Jay, welcome to the forum!
In a word, rust!! It's a disco, they can be really problematic in the bodywork department, rotten sills, door pillars, inner wings, boot floor, rear doors, various bits of chassis. Just have to be really thorough when you check it over.
V8's in particular: Anything from 70k miles upwards will be probably needing a new cam. The metal used was too soft and ends up giving a less than inspiring drive. V8's should be completly smooth and quiet, willing to pull and rev cleanly to 6k + (on an auto this is slightly different). Big problems occur through dirty oil, the engine will still run but can be choked to death internally, look for dirty valve gear through the oil filler cap. On an EFi like you are looking at, there can be idle problems, hot or cold start issues and general running faults. More often than not they are not difficult fixes (base idle reset, stepper motors etc) but the air flow meters can be problematic so be aware of the cost incurred (upto ~?100 for a replacement). Similar story with ECU's. It is not unheard of for 3.9's to have the slipped liner problems, be very vigilant on coolant levels, check for leaks and watch the temperature gauge carefully.
Don't let that put you off though. Everything is repairable and and with a V8 Disco, they are less likely to have had a hard life like a Tdi has.
