So lets say i picked up a cool turboprop plane from a buddy who hasn't flown it since 2010 thats around 15-16 years ago.

Do you just gut it and replace everything? Airlines, all new wiring?

It has Futaba 6v digital S9157 425oz servo that are now discounted.

I have never heard of airlines going bad. The ones that just came out of my jet when i "upgraded" to electric retracts and brakes were 20 years old and were still just as soft and functional as when they were new.

As far as the electrics, I would do a new RX and batteries. Depending on how much actual use is on the servos would determine if they get replaced. If they are still good and not worn I would keep them. But being 6V you will have to either use 6V batteries or add a voltage regulator of some kind. If you go new then everything can be high voltage.
With the age of the plane, look at all of the wiring If it is the old cheap #24 AWG wire I would upgrade it to all #20 AWG or better. Pull a few of the negative pins out of the connectors to inspect, if the copper is black, replace, if still copper or silver colored they are still good.

The only airline I have seen 'go bad' was the Robart orange air line & the purple air line. This deteriated to the consistency of cheese
both in models & new in the packet. Would crumble & fall apart when I tried to remove it from the nipples of cylinders & valves needing
careful use of a knife & cutters to remove it.

as long as it takes to get a set of electrics to replace the pneumatics...

if they are over 3-5 years old, i will remove the lines and clip the tube back about a 1/2\inch or so and re install. seems that the stretched connection part always deteriorate quicker than the bulk of the airline.

I might be an oddity, but anytime i buy anything of value that is used, everything gets pulled out of the airplane. Air lines, fuel system, wiring, literally EVERYTHING and anything flexible (air/fuel lines) get replaced and more often than not, the wiring all gets replaced too.

not an oddity in my shop....

used stuff, i(you) got no idea of it's past history/treatment.

exactly! I've bought more than a few airplanes that looked really nice, but under and behind stuff, wiring was twisted together and covered with electrical tape, had horrible solder jobs, cuts in the insulation, screws ran through it and all sorts of other issues. Sometimes i've even found major issues where bulkheads and other structural items had come loose over time and if the airplane hadn't of been disassembled as far as i took it, you never would of found the problems.

Had to replace the old Robart pink and purple airlines on my 25 year old EuroSport a few years back. Never had to replace BVM / DreamworksRC / McMaster-Carr airlines (all the same material no matter the color).

Nick