What is sufficient estimator?

An unbiased estimator is a person who gives a price for a service or goods and that person has no ulterior motives that would influence the price either way. A person who is biased might reflect the...

Adequate, enough, satisfactory, necessary, appropriate, ample, plenty, abundant, tolerable, acceptable, suitable, respectable, decent, fair, up to standard

It can be - depends on the purpose.

A synonym for sufficient is the word "enough."

Yes, a free income tax estimator can be as good as a paid income tax estimator. A tax estimator just allows you to have an estimate of your tax return.