I thought every ship has the Discover Scanner as standard now (as of the next release)?There's two different scanners for exploration.
Discovery - Basic / Intermediate / Advanced
Advanced gives a full overview of any system that's not already shown as explored on the galaxy map (The system map icon in the galaxy map is red, not white and you can't click it to open the system map (You could buy the data, if it's available)). Alternatively, if the system has a Navigation Beacon (near the primary star) drop into normal space and scan it to reveal all the same data the ADS would have given you.
Again, all three of these are about to be obsoleted, and included for free in every ship with the additional functionality of the DSS below...
Detailed Surface Scanner
Currently, will only give you a better cash payout on any object you target and fly towards in any given system. You have to get pretty close, and it takes time, which can be reduced by engineering it for range or speed.
The new DSS in the next patch adds a new mechanic for mapping planet surfaces and pinpointing anomalies and hands all it's current functionality over to the free discovery scanner mentioned above.
What might be the smarter move at this point is to start accumulating cash prior to the next patch so you're in a slightly better position to buy a decent ship - Cobra MkIII highly recommended, A-Rate the FSD and vastly improve the range you can travel per jump as some of the engineers are a fair way from your start point.
To get up to speed on what's coming, check out some of the videos on the official Frontier youtube channel, here. If you look in the Community tab you'll find links to some of the more prolific youtube content creators that offer in depth how to guides to get you up and running a little quicker than you might otherwise end up dead, a lot
Edit: A more direct answer: If you're not in a system, but you can already open and see all that system's data via the galaxy map, there's no point going there to try and make cash scanning it. It's already explored, so there's no point.
If a system is not explored, but you can buy the data, it IS worth going there with a DSS and pulling level 3 scans for all the objects, which you can then sell to any Universal Cartographics facility, as long as you travel outside the 20 light year exclusion zone.
If a system is not explored at all, the Basic Discovery Scanner has a very short range, only 500 Light Seconds, the Intermediate is 1000 LS, the Advanced is unlimited range but will also cost well over a million credits to buy (refunded when the patch drops). You might still be better off scanning the Navigation Beacon and getting the level 3 scans, or just avoiding those systems until after the patch.