1) The exhaust gas oxygen sensor (EGO or O2), or lambda sensor, is the key sensor in the engine fuel control feedback loop. The computer (ecu) uses the O2 sensor's input to balance the fuel mixture, leaning the mixture when the sensor reads rich and richening the mixture when the sensor reads lean.
2) Bushes. The front bushes are part of the wishbones, thus you will need to change the w/b arms if the bushes are bad. Some ppl have 'pwerflexed' these. Personally I find the ride too harsh. Powerflex make replacement bushes for many cars, they are made of a stronger material than standard and thus last longer. However having this done cost a lot more and the bushes may well outlast the ball joint which is also part of the wishbone. Front w/b are not expensive (I got mine for £110inc delivery and anti roll bar droplinks).
Rear bushes need to be changed quite quickly if they are worn as they can lead to rubbing on the subframe which will have to be replaced, this can be very expensive ! I have not changed the rears, but again powerflex do replacements. I will be doing mine in the next month or so, but others have considered this to be a good mod.
3) If you have no history with the car and it seems to be a little neglected, CHANGE THE CAM BELT ! Also change the tensioners, variator, balancer shaft belt and aux belt. Do not skimp in this area change the lot.
4) If its a gtv and its a 2.0 its a TS (twinspark) Alfa only offerred the TS 2.0 and 3.0 V6 in the UK, on the continent they had a 2.0 V6 turbo. I take it your in the UK.
5) Don't know.
In general they are fine cars with very good 'go cart' handling. They need no more general maintenance than similar cars from other makers. However the achilles heel if any are cambelts, bushes and oil. They can use oil and this needs to be checked at least weekly. Do not allow the car ever to run low on oil, personally I will never let a TS engine get to below 3/4 full. Use the correct oil too, selenia 10/40 or a good semi on the same grade.
Alfa gtv 2.0 ts