I bought mine with just enough cash to put in some basic pulse lasers, upgrade the power plant, FSD and thrusters slightly, and still have enough for insurance. Then I went to town and started gunning bounties and checking the BB for 'kill X pirates' types missions, or the occasional lucrative 'haul all this gold to X and we'll throw money at you' mission.
This was a while back, so I don't recall the numbers involved. I basically turned in bounties until I had enough to upgrade something, then bounced to a station to pick up whatever I was after that time around, then headed back out again. I also jumped into several of the Community Goals missions and used them to help bankroll the upgrades.
Yes, it took a bit, but I had fun doing it, and I'm only short 2 or 3 modules to have everything kitted out.