100,203 in word form is One hundred thousand two hundred and three.
Yes there are, but it really would depend on your definition of rare. There are degrees of rarity, sometimes using a 10 point scale ranging from "Most frequently seen" to "Highest rarity". Old hand written Bank of England banknotes predating 1775 are described variously as "few are known to have survived", or "none are known to exist". This last statement does not mean that they do not exist, it means that an expert capable of recognising them for what they are, has not seen one. The rarest of banknotes are in museums or the collections of very wealthy collectors. On a more realistic level, first and last banknotes in a series would be considered to be rare. These are notes with serial numbers ??? 000000 or ??? 999999, for example. Banknotes with all numbers the same ??? 000000, ??? 111111, ??? 222222, etc. would be considered to be rare. Banknotes with serial numbers prefixes that spell a word such as C01N or 01L. Banknotes with a genuine printing flaw, whatever that might be.