I am working on SharePoint multiple versions. 
One of the my customers is asking for the up-gradation from **SharePoint 2010 (foundation) to 2013**. 
I would like to know the below: 

 1. steps to upgrade the SharePoint 2010 to 2013.
 2. Impact on existing system which will affect solutions. 
 3. Sandbox solution
 4. Farm solution
 5. OOB solutions
 
The most important are my **Workflows** which are running for approval. The **application pages** and also the **event receivers** will may get affected. I have **search also**. 
What could be the procedure to handle all these.