I'll try and answer several different things, however my contribution may not cover all of your questions. Maybe several of us can take different chunks out of this. However, this info should be helpful for you. Here we go..
Opening A Seperate File:
ChDir "[Path here]" 'get into the right folder here
Workbooks.Open Filename:= "[Path here]" 'include the filename in this path
'copy data into current workbook or whatever you want here
ActiveWindow.Close 'closes out the file
Opening A File With Specified Date If It Exists:
I'm not sure how to search your directory to see if a file exists, but in my case I wouldn't bother to search for it, I'd just try to open it and put in some error checking so that if it doesn't exist then display this message or do xyz.
Some common error checking statements:
On Error Resume Next 'if error occurs continues on to the next line (ignores it)
ChDir "[Path here]"
Workbooks.Open Filename:= "[Path here]" 'try to open file here
Or (better option):
if one doesn't exist then bring up either a message box or dialogue
box to say "the file does not exist, would you like to create a new
one?
you would most likely want to use the GoTo ErrorHandler
shown below to achieve this
On Error GoTo ErrorHandler:
ChDir "[Path here]"
Workbooks.Open Filename:= "[Path here]" 'try to open file here
ErrorHandler:
'Display error message or any code you want to run on error here
Much more info on Error handling here: http://www.cpearson.com/excel/errorhandling.htm
Also if you want to learn more or need to know more generally in VBA I would recommend Siddharth Rout's site, he has lots of tutorials and example code here:
http://www.siddharthrout.com/vb-dot-net-and-excel/
Hope this helps!
Example on how to ensure error code doesn't run EVERYtime:
if you debug through the code without the Exit Sub
BEFORE the error handler you'll soon realize the error handler will be run everytime regarldess of if there is an error or not. The link below the code example shows a previous answer to this question.
Sub Macro
On Error GoTo ErrorHandler:
ChDir "[Path here]"
Workbooks.Open Filename:= "[Path here]" 'try to open file here
Exit Sub 'Code will exit BEFORE ErrorHandler if everything goes smoothly
'Otherwise, on error, ErrorHandler will be run
ErrorHandler:
'Display error message or any code you want to run on error here
End Sub
Also, look at this other question in you need more reference to how this works:
goto block not working VBA