A) Depends on the engine - The golf 3 had the old 1.6 8v (good engine) wheras the mk IV was available with a few very different 1.6 engines (old 8v, different polo derived 16v, and I think the fsi first appeared in th IV)

B) In the MK IV the electonic gearbox control systems will be very different - If I remember correctly the electronically controled auto gearbox was introduced with oil sealed for life (which is probably why it's knackered)