I'm British, but I can answer for the UK and US:
$100 — a hundred dollars or one hundred dollars?
"A hundred dollars" is how I'd say it in speech. "One hundred dollars" is how I'd write it on a cheque.
$201 — two hundred [and?] one dollar[s?]?
In the UK we'd say "two hundred and one dollars". In the US, they might say "two hundred one dollar[s]".
$1500 — fifteen hundred dollars or one thousand five hundred dollars?
"Fifteen hundred dollars" is how I'd say it in speech. The more "proper" way to say it, and the way I'd write it on a cheque is: "One thousand, five hundred dollars" (never "one thousand and five hundred dollars").
$1525 — fifteen twenty-five dollars or [one/a] thousand five hundred twenty-five dollars?
I'd never say "fifteen twenty-five dollars", I'd either say "Fifteen hundred and twenty five dollars", or "one thousand, five hundred and twenty five dollars". Americans might skip the "and".
For a non-English speaking country, say the number fully using "one", for the sake of clarity. In some countries though (such as the Netherlands and Norway), the use of "fifteen hundred" etc. is the same in that language too.
It's not "as will": it's "such [...] as"
One should try to post such questions as would give everyone an opportunity to learn something.
When going through the casting process one must select such performers as wouldn't make the composer, Giuseppe Verdi, turn in his grave.
Young architects should design such buildings as will present a luring opportunity for a cityscape artist rather than bore the hell out of him.
Best Answer
In the United States, when writing a check, it's customary to write and 00/100 or and no/100 or and xx/100 before "dollars" to indicate that no cents are to be added beyond the indicated number of dollars. Sometimes people also follow this custom when writing a contract. The "/100" refers to cents, since there are 100 cents in a dollar. Sometimes people write and no cents after the word "dollars", or the word Exactly before the (verbal) number of dollars.
The purpose of writing out the number of dollars in words, in addition to writing out the number in numerals, is to make it difficult for one party or an accidental mark to modify the number after the check or contract has been signed. Often one numeral can be changed into another by adding a stroke or two with a pen; it's much harder to modify spelled-out words without being detected. Similarly, explicitly writing that there are no additional cents prevents the recipient of the check from increasing the amount by adding "and 99 cents" after the word "dollars".