Scanning your car BEFORE you start modding is a good idea. The a pre mod scan you will have a baseline and be able to see what areas need to be addressed. A lot of stock cars have knock to begin with and it is helpful to know this ahead of time. It's unlikely on a stock 04 that you have any DTC's. IF you do, usually they are ones that don't matter...

No a dealership or 99% of the shops will not be able to scan what you need scanned. Befriend or pay another GP owner/vendor to help you out.