Maintenance cost isn't high because spares are easily available and a Golf is a reliable car.
Performance: There are 2 kinds of performance:
The 'standard' kind: It's a good performer. They work well in U.A.E heat and are fuel-efficient.
The 'PUT-A-SMILE-ON-YOUR-FACE' kind: Bang for the buck I have to say. Although they are not very fast, they are certainly faster than most cars on the road.
Modifications: Zak is the man to talk to. Although I think modifications are easily available. Performance group might have kits to offer and engine mods are available too. Power Curve trading has superb bodykits for the Golf that you are looking for (00-02). Speaker systems, lowering kits and wheels can be found from anywhere. Digitec does ECU tuning for the GTi.
It is a durable car and withstands abuse from the owners very well.