Will it be possible to smoothly update a slipstreamed XP SP3 RC1 to the final SP3 or do i have to manually install the final SP3 and my C:\Windows folder will be filled with SP3 Install/Uninstall files.

You really should always update a new source. Especially with using a beta SP.

Will it be possible to smoothly update a slipstreamed XP SP3 RC1 to the final SP3 or do i have to manually install the final SP3 and my C:\Windows folder will be filled with SP3 Install/Uninstall files.

Only use release candidates in a test environment. My suggestion, wait for the final release.

Is it possible using repair install with a slipstreamd SP3 nlite XP on a SP2 nlite XP(same preset) and gain the same end results as a new installation regarding C:\Windows folder size, etc?

If not it seems like a waste of time to install a nlite xp sp2 now instead of waiting for sp3.

Waiting for SP3 will give you a cleaner install than upgrading SP2 to SP3.