Hi
There are many threads on EoS - and elsewhere - about this, but in essence:
1. It will cost €700 - €1000 to get the reg changed. Pay someobne to do it for it it's far easier than wading through the the forms and beaurocracy.
2. You need to make sure that tyres are "paired" on each axle. Note necessary in UK but essential in Spain.
3. Can't say about insurance levels - we've been here too long to remember, but are probably not much different for a left-hand drive car. Right-hand drive cars attract a premium (issues with mirrors, overtaking etc)
4. Yes - second hand cars are more expensive in Spain, but thats due to a) mileage is usually a lot lower than UK and b) there will be significantly less body work problems as climate is much "nicer" for cars. Although some old ones have serious paint work problems due to the sun.