Pro Tips:
1) As in real life, taking out massive loans is always a terrible idea.
2) Avoid girls with Cantankerous, Obnoxious, Greedy, Egocentric or Crazy (which, sadly, is about 90% of them). The best passive stat is Just for Fun, as it lowers their costs for shooting every type of scene and thus increasing your profits. The next most important stat is their Likes chart, and making sure it lines up with what you want them to do. For example, if your want to do straight Boy-Girl scenes, then sift through the rubbish to find girls with a Like for that at 75% or higher. So long as they like what they're doing then Happiness loss will always be minimal. Although it may seem like a good trait, Social Butterfly actually turns into a burden because they will spam you with new models damn near every day and, unlike your PAs, they have no filtering as to what kind of useless flesh-lumps they send your way.
3) All the special NPCs are useful as PAs except for Maria (the Maid), though some bonuses are worth more than others. Right away you'll want to recruit Jenny (the gym girl) to keep your own Fitness stat from rotting away each day. Next should be the nurse from the hospital to cut the costs & time of rehab - this isn't just for lousy hookers, this also affects plastic surgery, which you'll be doing a lot of once you've got good money coming in. Alice (the punk chick) gives a big marketing bonus, Lena (the cougar) trains up your other girls sex stat for free, and so on. The library girl (who's name I forget) shaves 5% of the salaries of other PAs, but that's not really a big deal. She & Maria are "expendable" in that regard and you may want to just keep them as porn models. All the rest, however, you definitely want to keep on as PAs, period.
4) Now that websites seem to be working again, there's an easy, winning formula here that doesn't require much work at all. As always, start off with disposable hookers but before you even do that think ahead as to what kind of websites you wish to have (i.e. what genre) so that you can sift through the hooker pile to find girls that match your future goals. Why do this? Because every video you put on the PornTubes creates fans, and fans that don't match your website do you no good in the long run. Of course you can still do the thing of selling off the rights for flat-cash, but still, hookers are common & cheap enough that you should never be that desperate.
5) Don't even build a porn studio. Just a photo studio should be enough and even that is optional. A half dozen girls with Likes for Photos in the 80% or more range can cycle through photo sessions pretty much forever, and while it isn't much money, it costs your MC zero energy, zero happiness and zero personal time to do this. And with good enough models and crews it actually can become decent money after a while. Best of all, the photo studio doesn't cause equipment decay.
6) You can keep on shooting at-home or in-office amateur porn all the way through to the end-game. Yes, a full studio with a max level staff and top price gear will make more money per scene, but scene money isn't where the end-game is. The end-game is in your websites, websites are based on fans and fans do not require top tier stuff to acquire or keep happy. You do your two-per-month scene quote for a website with your cell phone camera and keep your customers just as satisfied as you would have with a full studio scene. So your real end goal isn't cash - it's fans, and getting more fans is definitely more of a quantity over quality thing when it comes to doing scenes. More fans equals more subscribers equals more free cash per month.
7) Once you've got all the special NPCs you'll still need three more top tier PAs of the more generic NPC kind: Analytics, Marketing and Talent Search are what you want, here.
8) Don't overlook security guards until you go through to cops' story. Once you hire them, however, the two of them alone will provide you with more security than you'll ever need.
9) Don't over staff yourself, as staff are hands down your biggest expense. Like I said, you might need a photo studio but that's it. Even then, once you add in all the PAs and special NPCs, you're still looking at a staff of nearly twenty. That's expensive, especially once they rank up to the top. Therefore I advise stopping your special NPC hunting after Jenny and the nurse until you've built up enough website subscribers to float additional salaries. You should never assume that you can just film enough scenes to pay all your bills each month, because those bills are going to be higher than you think.
10) Your website is your real paycheck. Scenes are just a way to pad your fandom and increase subscribers. Scene money is just bonus cash, not something you should be gambling your entire business viability off of.
Bonus Tip: Once you're rolling in money consider contracting your lesser used models and running them through plastic surgery and training until they're completely maxed out. The end results can be impressive. Also contract out your photo models whenever possible. Why them instead of porn models? Because photos cost the least amount of happiness and thus their downtime is minimal as well. In the end you'll make more money off them because you can, under contract, shove them back into that studio non-stop and never have to pay them any extra for it.