Upgrading SharePoint from 2010 to 2013 with Farm Solution

Question

I am working on SharePoint multiple versions.
One of my customer is asking for an upgrade from SharePoint 2010 (foundation) to 2013.
I would like to know the below:

1. Steps to upgrade SharePoint 2010 to 2013.
2. Impact on existing system that might affect solutions.
   3. Sandbox solutions
   4. Farm solutions
   5. OOTB solutions

The most important are my Workflows which are running for approval and there are application pages, event receivers and search that may get affected.
What could be the correct procedure to handle all these.

Update (17th Feb 2014):
I have SPD workflows and also Visual studio workflows.
I have application pages developed in visual studio.
I have SPD forms (Custom Pages developed in SPD 2010).
I have Timer jobs and event receivers developed in Visual Studio 2010.

Answer 1

Steps to Upgrade from SP 2010 to SP 2013: Make the site in SP 2010 readonly. Using SQL Management Studio, perform the backup of Content database and service database in SP 2010. Restore the databases in SP 2013 SQL server. Attach the content database to a site collection in SP 2013. Similarly, attach the service databse to the corresponding service in SP 2013. For more info please refer: http://sharepointnadeem.blogspot.com/2013/12/sharepoint-2013-upgrade-from-sharepoint.html

UPDATE

SharePoint 2010 workflows are supported in SP 2013 also. The only issue I faced while doing migration of SP 2010 site having workflows was about workflow activities:

The type or namespace name 'AssemblyName' could not be found (are you missing a using directive or an assembly reference?)". at Microsoft.SharePoint.Workflow.SPNoCodeXomlCompiler.LoadXomlAssembly(SPWorkflowAssociation association, SPWeb web) at Microsoft.SharePoint.Workflow.SPWinOeHostServices.LoadDeclarativeAssembly(SPWorkflowAssociation association, Boolean fallback) at Microsoft.SharePoint.Workflow.SPWinOeHostServices.CreateInstance(SPWorkflow workflow) at Microsoft.SharePoint.Workflow.SPWinOeEngine.RunWorkflow(SPWorkflowHostService host, SPWorkflow workflow, Collection`1 events, TimeSpan timeOut)
at Microsoft.SharePoint.Workflow.SPWorkflowManager.RunWorkflowElev(SPWorkflow workflo...

Opening the workflows associated with these lists from SharePoint Designer resulted in the error: Could not deserialize object. The type could not be resolved.

Resolution: Web.config files needs to be modified. For SharePoint 2010 custom activities to work in SharePoint 2013, the “authorizedType” node needs to be defined in a new sub-section “targetFx” under the “authorizedTypes” section.

I also had plenty of event recievers and they all worked very well.