Requiem?

I have been interested for a long time in writing a Requiem. I tried to google some information but couldnt find anything.

Can a Requiem be any instrumentation? So it is just the text that classifies the piece as a Requiem?

Yes. As long as you follow the liturgy and you use a voice, then it is a Requiem.

You can also (technically) add movements of English text or your own text or other poetry. But, it is not canon and will not be a "true" Requiem by definition.

But, I know some Reqiuems that do that. hahah.

A Requiem is a Mass, is not only funeral music, is a sung mass with funeral music, so the text must be followed, without alterations, Mozart's Requiem would be the perfect example.

Compared with the traditional Mass form, Requiem has additional sections that require music, in short.... I'd say you follow the Mozart structure, (text and movements) just with your own music:

http://members.optusnet.com.au/charles57/Requiem/lyrics.htm

in case you want it to be performed in an actual Mass, you'll need some other musical segments.

as a personal note, I say "Sanctus" must be always imposing, no matter if the mass is a funeral, is Christmas or whatever, in a requiem it can temporary break the funeral aspect, no sadness or calmness, God is God in life and death.

The Sanctus in John Rutter's Requiem is a brilliant example of ^

Or you could be like Takemitsu and totally write a "Requiem" for string orchestra. Also Rutter's Requiem and everything else he's ever written sux

^ Respectfully, I disagree with the latter half of your statement.

^ I respectfully agree with the whole of your statement.

^ Respectfully, of course, I can only assume you mean that you don't think ALL of Rutter's work sucks; yet like all composers, there can be room for pieces that just are plain scraggy.

...

In that case, I do agree with you as well.

I would say that it is usually the text that classifies a piece as a requiem (but you can do anything nowadays). Purely instrumental pieces can be called an "elegy," "Trauermusik," "funeral music" etc.