I responded in the corporate section, but 7 to 8,000 is not unreasonable to expect when maintained properly. Getting the bore scope done is important so you know what you are starting with and know the previous owner was not constantly over-temping the engines. The MORE program sets out maintenance procedures that make the engine actually certified for part 135 to as much as 8,000 hours. Don't spend the money on the actual STC for MORE, but research it and maintain your engines the same way and you should have good results. Just as an example every 400 hours some of the things that are done include changing fuel nozzels, compressor wash, re-calibrating all the engine gages, propeller balance and vibration analysis, and oil analysis. Track your engine parameters each flight to monitor trends, mostly for the health of the hot section. The rest of the engine is pretty bullet proof - the only compressor or power section failures I have seen were due to FOD.