1) hit or miss, some cars go loads of miles with air suspension, others dont.
2) Doesnt make sense replacing good parts
3) £1-1.5k is about the norm
4) Get a service done, no point getting MOT done. Reason being - that if there is an issue, the MOT will show as a fail, whereas a Service should point out any issues. Also, the MOT doesn't check as many items as the Service does, its just to ensure your car is road safe thats it.