An advantage to hiring the higher tier mercs is you can then take down the Ceilor Forest Raiders and the Highwaymen of Darkmere every 3-4 days. That's a lot of chances at coilgun drops, not to mention all the rifles and other goodies you can find. That's how I get my house upgraded ASAP, which extends a lot of other benefits.
I've still not made up my mind about the best sex perks myself.
Yeah I was just thinking after upgrading to them in my current game that it might have been better to just go ahead and grab them asap, although I'm not sure about getting Bud super early since his aim is a bit low and he eats up a bunch of time with his attention whore trait. I think I'll still wait on him until after I finish all the classes. Doyle is better until Bud gets his aim high enough to really get good use out of a coilgun IMO.
I do think burning through all 5 classes and a bunch of books early on makes a lot of sense though. It's by far the biggest boost to stats you can get along with the passive bonuses you mentioned from the house upgrades. Daria's Gift in particular is extremely helpful early on for seduction.
I ran out of money with the low tier mercs while I was taking classes so I got stuck at level 1 upgrades for an annoyingly long time (about a month and a half). I wasn't spending much on salaries but I also wasn't making much via combat.
I could have squeezed in a couple of extra upgrades if I hadn't got the aim implant, but to me upgrading my ranged to basically guaranteed hits by early in week 2 is worth it. It would have been better to spend it on the high level mercs first though and then get the aim implant. I tried setting up Rebecca as both a maid and as head maid to see what effect she had on my passive income but I think I probably need to train up her domestic skill before she'll give me much benefit there.
I might try a new game with Arouser + Master of the Ropes so I can really focus on breaking and training my slaves immediately before I've taken any classes or books. With low starting bondage I haven't really bothered to do any training at all for the slaves until about the 2nd month (other than letting a slave join me when I'm training my own stamina). It would be nice to for instance break Rebecca's will immediately, train her domestic skill up around 80 or something, and then work on getting her willpower back up again later. Then again, if I'm going to really go to town with bondage it might be better long term to get Master of the Whip since it seems like Flagellation is harder/more expensive to increase than bondage. At least with bondage I know I can raise it for free via aggressive petting, albeit slowly.