Why in the name of Bob do you feel like you need to spend that kind of ISK?
One skill injector will turn a brand new character into a decent extraction alt. Or you can just spend a week training up CCU IV and IPC IV.
You only really need command centre upgrades V on factory planet managers.
How long it’ll take you to manage that many planets is a good question. Back when i did a lot of PI, i reset my extraction planets once a week and had to feed my factory planets every second day. From memory, i collected resources from the pocos every two or three weeks and from the factories every four days (use the poco as extra storage for materials to minimise the amount of time you spend derping about.)
I spread it around so i didn’t have to do it all on one day. I spent maybe 10 minutes a day on my PI over it was all setup and another 10-20 minutes a couple times a fortnight picking stuff up. It’s very hard to give you any concrete estimate as there are a ton of variables.
I’d suggest taking it a little slowly. Train up two alts as extraction alts. Make a few extraction setups, try a basic factory setup.
PI is a huge PITA to get setup, the real payoff is you’ll still draw income from that setup until you move out of that area of space.
I strongly urge you not to blow 70b straight up. Go a little slowly, figure out how to be efficient in your area of space.
As for what to make: i used to make nanite paste. I’ve also done well in the past making fuel block components. I’ve never done P4 myself, but the returns could be worthwhile.
In any case, here is my standard, obligatory repost: Meph's Magical PI Setups for the Interminably Lazy