Need help with codes Please 1996 SL320

[tr4me]

After spending a couple hours researching the forum, putting together my new LED & switch code flasher, pulling the cover off the 38pin diag. plug, printing pictures of diag. plug and pin identification charts, I finally try to read whatever code has brought on the check engine lite, and car is running poorly.

The diag. connector on my car has empty holes on #1, #3, #19, #10 along with most of the other supposed female 'pins' of the diag conector. Is this because this is an OBDII car?

Do I need to buy one of the $80-$100 OBDII code readers from Advance Auto?

What are the chances of it actually retrieving the DTC on my car?

Background info:

1996 SL320, 100k miles, very well maintained, second owner from 40k to present.
It was running perfectly when my wife shut engine off at the bank, when she returned and cranked up, CEL came on, engine began running roughly.
I have pulled airbox and checked filter, and MAF visually, look like new. Visually inspected all the vac lines and hoses I could find, no problem found.

Thanks for any insight,