Hey there CMDR,
first step into long range is mass optimization. As you already know, upgrading FSD = more range. But FSD alone won't give you much unless you downgrade your overall mass. All of your ship's modules have different grades, stats. And mass. General rule for long range is:
- FSD the biggest you can afford (A-class is the goal)
- fuel scoop the biggest you can afford (as above)
- everything else D-rated (lowest mass)
- (optional for credit income) adv discovery scanner + surface scanner. Honk-scan every system you enter to know it's composition, detail scan earth-like, water worlds, high metal content + main star. When you return, sell data for extra income (TIP #2: save it for now, more details with Engineers).
Since you plan on passenger missions you *may* drop weapons and go unarmed (lower mass, but no means to defend against anything. Run and hide option). You can go without shields as well (even lower mass, more jump range). Bit hardcore option but perfectly doable. I had my share of shieldless exploration, just you need to be a bit more careful.
Next step is upgrading your ship to something better. Adder is fine ship, take a look at T-6. Next are Diamondback Explorer and Asp Explorer. Above rules for outfitting apply.
Next step is engineering, like Felicity Farseer. Tho her first upgrades aren't thrilling, G5 is what you want. When you decide to visit her - remember about exploration data - sell it to her. Not only you get credits but you will progress with her reputation. With enough data (5M worth should do the trick) you will skip G1-G4 grind and will have G5 FSD rolls available off the bat.
Does engineering is worth? Let me answer with example.
DBX, stock. 20+ LY range.
DBX, stock exploration. 33 LY range.
DBX, stock exploration with G5 FSD roll. 50+ LY range.
DBX, G5 FSD, extremely low weight (empty, shieldless, pretty much just seat and engines). 60 LY range.