Another question, is it possible for the zero start to train a concubine as the first slave?
Tried it. Zero start, guild contract for concubine. Slave was Plain, Feeble, Callous, Reserved, Stupid, Uncertain, Haughty, Voluptuous. Completed contract at day 58, receiving 205 sparks. Incurred 170 spark penalty for being late.
After repaying debt, 83 sparks remaining, 2 days until next arena battle, 2 days until next billing, and with a spare slave already owned (in cryo). Master stats (skipping F-): Weak, Lustful, Passive, Rumored, Houseboy, Uncultured, Brawler, Petting D-, Oral D-, Penetration D-.
At day 58, slave had 3 stars devotion, no fear, no despair, 3 awareness, 4 taming, 5 habit. Healthy, Insensitive, Reactive, Stupid, Uncertain, Unashamed. Took a while to raise her endurance and lower her pride, which contributed to the late penalty.
Ended up taking a loan in the last few days because needed 2 more sparks for threesome lessons. (2!)
Could've avoided the loan if I had gotten rid of the second slave I was keeping in cryo that I bought from the market for 20 sparks in the previous decade. The slave was worth 13 sparks to the butcher and earned 50 in the arena, so in total I could have made 43 sparks profit from her (63 - 20) if I got rid of her immediately after the battle. However, I kept her hoping to use her for the next guild contract, and that decision ended up costing me 18 sparks in cryo fees and (arguably) 100 sparks in loan fees. Even so, she will earn another 50 sparks in 2 days and another 100 the following two decades, so keeping her is still profitable in the end, but unfortunately the moneylender got a large share of it.
Still, I consider this a good start. Out of debt, out of contract, some master skills developed, and next slave already owned.
Alternatively, you could continue training. The slave had C- fame. By waiting one day to turn her in and training her a bit more, I was able to get her to C- and received an extra 40 sparks. But I spent some sparks to achieve this, so it was a net gain of 22 sparks.
C+ was possible after an additional day, by having her do the accounting and then giving her a few lessons, her intellect raised to C- which satisfied the basic attributes requirement. Day 60, received 310 sparks (105 more than day 58, 65 more than day 59), but spent more to achieve it (including a 12 spark fine for sloppy accounting), so ended up with the same net gain of 22 sparks. Also, day 60 is arena battle day and the cryo slave hasn't rested, so before turning in the slave, you'd want to win the arena battle with her (which is doable).
If you kept this slave and continued training her, you could reach B- (by getting her fame up with two more arena wins and maxing 2 common skills and 1 sexual skill category) before the loan deadline. A B- slave would be worth an additional 575 sparks (compared to C-), plus she would be earning ~70 sparks in the arena for those two decades as a champion, and so even with the additional speed penalty for waiting another two decades (~200 sparks), this would be a net gain of 300 sparks (minus other expenses, of course, which the ~70 per decade from arena would help to cover). If you got her to B+ (which could be done in the same timeframe if you focus on raising her temperament with lots of orgasms [with the side benefit of raising the master's strength]), she'd be worth an additional 750 sparks (compared to B-).
Lessons? Being late is increasingly excusable as you train higher ranks. Taking a loan as late as possible (or never) gives you a chance to take a contract slave a lot farther. If you're training a concubine, aiming for at least B+ rank is a good idea because you've already done most the work of sex training for that rank. It is possible to succeed even if you start with a slave that has less than optimal attributes (in this case low endurance, high pride).
Incidentally, this slave was a loli, which made the arena particularly challenging. She ended up with a specialization in dual wielding after trying and failing with other styles, but other slaves will have a different experience.