Compilation has some limits for some newest packages, thats why people working on maemo leste. But with a lot of effort you can still update harmattan or compile packages which doesn't exists in harmattan.
You will need a sdk there you have two choices qt/madde(windows example) or scratchbox. I would recommend scratchbox for compiling packages but it exists only for linux. For qt gui applications I would recommend qt/madde.

When you are not so experienced I would go for a vm image.
I personally created two and never tried something different so I can only explain my images.
The first vm image which has already a toolchain configured for harmattan, has both qt/madde and scratchbox installed, which makes it really huge.
The second small vm image has only scratchbox installed there you need to configure the harmattan toolchain.




Debian archive look for the dates:
http://archive.debian.org/debian/pool/main/

You could try packages from N900:
http://maemo.org/packages/

You could try from Ubuntu look for the dates:
http://old-releases.ubuntu.com/ubuntu/pool/main/