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She has served me well for 12 years but is starting to cost money -new cam belt and new clutch last year.
Goes well but body work not so flash.what is a reasonable asking price for a trade-in when I buy from a dealer which is my preference?

This is very difficult to answer without seeing the vehicle, especially as you say the body work is not so flash.
Depending on the model (and there are numerous Corolla models of this year), the Red Book valuation guide suggests a trade-in value of a vehicle in average condition would be around $1,500, but this could vary considerably depending on what you are buying.
A dealer will "over-trade" if you are buying a $50,000 vehicle that he has a big margin in, but if you are only spending $5,000, there is less room for him to move.
Ultimately, the most important figure to you is the change-over figure (ie; the difference between your trade-in and the car you are buying).

Thanks that seems reasonable.
Should I avoid demonstrator cars which seem to be really cheap despite low Kim's
What kms are excessive for a 1-2 yr old vehicle?

Demonstrator cars represent great value, so no, you shouldn't avoid them. At this time of year there may still be some 2011 demo cars around, so the price would need to be less than a 2012 registered demo. The advantages of buying a demonstrator are;

1) Reduced price from new retail.
2) In most instances, the remainder of new car warranty and any service plans offered by the manufacturer will apply.
3) Often they may include extras that would otherwise be charged for.
4) When it comes time to sell, they're still almost considered as 1 owner cars. You will often see them advertised as "1 owner + demo".

In answer to the question of kms, the average NZ motorist travels 14,000km per year. Much over about 25,000km per year is considered quite high by NZ standards.