Alright so the game doesn't really do a good job of explaining itself, but having just beat the game here's what you need to know
You are given 1 potion at the start of the game. This is essential. Each potion heals for 150, which at base level is another 15 floors, with upgrades as much as 30. Buy your first girl, run through the dungeon (take every gacha, there is no downside), use the potion when out of HP, and go till you can't. Get as many upgrades as possible and use all your modification points.
Once you return, sell her. The next day, but a new girl and 2 potions (they cost 1500 each, since it's unlabeled)
repeating steps 1 and 2 on day 2 with your extra potion should raise her sell value enough that you can afford some upgrades. I'd recommend going for the increased potion stock upgrade first since it's cheap and pays off quickly, but pick what you'd like bearing in mind most girls cost a little less than 700 and you'll always want to buy max potions, so leave yourself cash for the next run.
Doing it once more should blow past that first debt payment, and from there now that you have the loop down the game is incredibly easy if not terribly engaging.
Breast and uterus upgrades only appear on floors 1-50
bladder and mammary on floors 50-100
Penis/scrotum 100-150
Anal can appear on floors 50+
Floors 51, 101, and 151 are always the navel upgrade
Mind break can appear on any floor
150+ anything can spawn
The gacha can:
randomly spawn any upgrade creatures (not sure if the gacha needs to be upgraded for this)
Heal your current girl
Give money
Give points
I beat the game in less than 2 hours, and while I like the content, it's severely lacking in game play. With no actual H scenes either, unless you're just REALLY into TF content, I'd pass.
The last debt quota is 200,000.
