We're planning to upgrade an existing farm from 2007 to 2013. I learned that the best way and maybe the only way to achieve this is by migrating in two steps. 2007 to 2010 and 2010 to 2013. The actual farm is a sql-server(2008r2 64 bit) and a sperate sharepoint-server(web-frontend, etc....) I want to keep the sql-server as it is, if possible and we want to install a new 64bit host for sharepoint 2013. We will design a new branding for 2013 afterwards, so we need to keep the content, but not the layout. There's so much about migration available on the web, but Can I find something like best practises?
You are correct in that there isn't a direct path from 2007 to 2013. You will need to upgrade 2007 to 2010 first and then upgrade 2010 to 2013.
My suggestion to you would be to setup a new virtual environment (you could even use the demo VMs you download from Microsoft) for 2010 and 2013. Then using the database attach method, migrate to 2010 and then to 2013.
Once you have your content upgraded to 2013 and working, migrate it to your production 2013 environment.
If you’re planning a move from 2007 to 2013, consider this: as a move from 2003 to 2010, the pathway is through the interim release (2007->2010->2013).
Microsoft has defined a support plan for the upgrade from 2010 to SharePoint 2013 preview, but no support is offered from 2007 to 2013 since 2007 Mainstream support has already ended in October 2012. An incremental upgrade from 2007 to 2010 will ease your upgrade to 2013.
As previously stated there is no direct upgrade path from 2007 to 2013. You have to go via 2010.
Unfortunately that is where the advice can really stop. Migration is a very complex activity and there are a lot of factors involved.
If your environment is very much out of the box with no customisations then you should have a fairly smooth ride. However if you have invested in custom development, SharePoint designer customisation, branding updates, third party add-ons then you have some testing to do.
I am just exiting a project migrating a 140,000 user, 100 Site Collection Intranet and 10,000 team site migration from 2007 -> 2010 with terabytes of data and it has taken around 21months from start to finish and a team of over 100 people! It was a resounding success but took a lot of work.
Depending on the size and complexity of your environment it might be worth engaging a partner with experience in this activity.
If your environment is small enough, then buildup some Vm's give it a go and see what breaks!