PPRuNe Forums - Aero Vodochody L-29 questions

I would love If anyone could enlighten my knowledge about this trainer jet and some general stuff about frames, conservation.
This is some brief data on airplane:
It was standing out on the open in Estonia for 14 years, although everything still works, a/c has 4hrs SMOH since they 've been doing sporadical checks on her.
My question is:
1.How bad can a skin and frame get (corrosion),If the plane wasn't flying but just being static on the airport and Is that repearable?
2.
4.2. Structure - Fatigue State

A manufacturer’s communication states that the airframe life is nominally 5000 hours provided that all airframe overhauls have been carried out.4 All such overhauls have been performed by factory agreed agencies for 194555 with the last one at 2010:10 hours in March 1984. An extension beyond 5000 hours is permissible on an on-condition basis. The maximum interval between overhauls is 1200 hours. This information is derived from the logbook front page for this aircraft, which is series 9.

The remaining airframe fatigue life at permit application is therefore:
5000 - 2715: 20 = 2284 hours: 40 minutes

Remaining airframe time before overhaul is therefore:
1200 - 705:10 = 494 hours: 50 minutes

I don't understand the part about the airframe overhaul? Can that be done and is it costly?
Also, the entire system on this plane is run by either electric system or Nitrogen, even brakes. Is that safer and more reliable than hydraulic system?
And finally, I saw some ex-Eastern european planes (ex-military) having to apply for Stage III approved thing?What is that exactly meaning?

Thanks for any input
BD