You can use the ProcessStartInfo class to separate your arguments, FileName, WorkingDirectory and arguments without worry for spaces
string fullPath = @"C:\FOLDER\folder with spaces\OTHER_FOLDER\executable.exe"
ProcessStartInfo psi = new ProcessStartInfo();
psi.FileName = Path.GetFileName(fullPath);
psi.WorkingDirectory = Path.GetDirectoryName(fullPath);
psi.Arguments = "p1=hardCodedv1 p2=" + MakeParameter();
Process.Start(psi);
where MakeParameter is a function that returns the string to be used for the p2 parameter