Do you want to end up with a font in the end, or just some letters with texture as vector or raster images?

I want to end up with a font in the end, a .ttf or .otf :)
Otherwise I know that I could just do it as a layer style in photoshop, but the client really wants it as a font file.

Since you said it was an existing font already: Make sure the EULA allows modifications! Many don't, and those few that do rarely allow you to sell the results.

You'll need some kind of font editor and a method of creating (tracing?) vector files from the noised-up letterforms. From there on it's just a matter of copy+paste. Off course, if you want it to look beliveable, you'll have to figure out how letterforms are distressed in real life, create multiple variants of each letter (informed by the real thing) and write some Opentype code to put it all together.

I would suggest making a high-contrast Photoshop file, importing it into the background layer in FontLab or Fontographer, auto-tracing, and then optimizing and/or cleaning up, if there are too many paths.

You will need to experiment to get the scale right.

I believe there was a Typophile thread about the limits to the number of points/paths in a glyph or font.
There is also information on importing bitmap images into Fontlab etc., to get the size consistent.

Wow thanks for your help, it is greatly appreciated! Really good for getting me started, your help is much appreciated! I can post the result here later if you are interested in seeing it.

If you have the thread-link that would be good to, or I'll just try and search for it :)

interesting with the eula, is that only for modifying the font-file or is it regarding all customization or only for making it a .ttf and selling to a client?
Because I am not quite sure the client will actually receive the font it will be used on a interactive campaign and I am not sure the client will need to use it after that.
But that can be checked on the website of the type foundry of the font right?

Anyhow first thing seems to be to buy a Font Lab License and then make the font in photoshop!

Again thanks a lot for your help, this community is so awesome!

Quite new info: http://typophile.com/node/81812

I don't think that building a font the way I described would contravene a font EULA, because you are modifying typography created by the font, not the font itself. The act of making a bitmap file destroys the font outline BCPs, which are what's protected.

That's particularly so with the noise filter, which is unlike most if not all other Photoshop filters in that it produces a different result every time, with the same settings.