Right lets start from the beginning. Its an 8 year old car. It WILL have faults! End of - parts do not last for ever.
Bushes are rubbish?! 8 year old car - these will go. They don't last forever.
A sensor has gone - again an 8 year old car
Seats don't move on their own?! My back rests do but the seat won't slide unless you push them. This is the same on brand new cars too. You bought the car second hand - you don't know how many times they've been used. At the end of the day those seats could have been used thousands of times - its a mechanical movement - it won't last forever! Needs maintenance.
Air bag light - who did it? Sounds like they didn't reset it or did the job incorrectly.
Seat - I'd put up with it. It'd cost a couple of hundred at least to fix it - may as well just push it... Not like its loads of effort.
Leak - fold the seats down, sit someone in the car and spray around all the doors, panel joints etc with a hose pipe. This should reveal the source of the leak to you. There's a possibility its the switch that causes the window to rise when you shut the door making the window stay open...
Just remember its an 8 year old car and it will have faults! Its more worn than a 4 year old car, hence why its cheaper to buy it