First of all, I second all points @Digitizer and @Harry are making. I would say that even after culling the archive and having just one third of the initial volume to scan, it’s still a very big undertaking. My take is that you certainly need to go camera scanning route which is inherently many times faster than any other type of scanning. I would say that you may want high end equipment just because the high end equipment is much more sound mechanically and thus allows for less focusing and better throughput. It also should have a decent resell value.
Slides should not be a problem as what you see is what you get. Yes, you will need to play with backing light to optimize the result, like having magenta filter helping with color fidelity, but all of this needs quite a bit of trial and error before you start getting consistent results. I would say that you should avoid measuring exposure for each slide, but rather adopt single base exposure and then bracket all slides as a rule. You will get three times the volume, but you are guaranteed that one of three will be good enough and you will be able to HDR process especially problematic ones.
The dust is the serious issue as well as film curling . I typically recommend to remove slides from paper mounts - there are good tips around how to do this efficiently , and then clean slides without mounts, and remount them in cheap thin plastic side-mounting mounts like Pakon or HAMA which will keep them all of the same thickness and will protect them from dust accumulation forever. The scanning of slide of same thickness will be much faster and reliable.
With camera scanning you will not have issues switching between 35 mm and 120 and 4x5 provided you have large enough backing light.
Keep in mind that for 35 mm you actually don’t need higher resolution than say 30 MP . For 120 and 4x5 you decide what resolution you want and if you actually need more than 30 mp even if image has more info than 35 mm frame.
this decision is important as it informs what sort of camera and lens you need. The heavier camera and lens, the more durable and rigid rig you need and that will drive the cost up significantly.
Ideally , if you can find someone in your area who already doing this, you can ask for the facilities tour to get first hand idea how people do it IRL.
This is big undertaking and i would go in steps before committing large amount of money and then realizing that process is essentially so boring that you may run out of willpower to finish. I am serious - once you start doing things and it becomes routine you will need certain resolve to bring this to conclusion.
The following device is probably the best for your needs: Let's see your DSLR film scanning setup! - #23 by ChrisCDSMedia , though it costs around US$3000, but still it is probably the best one performance-wise. Look at all other setups in that same topic to get an idea what can be done.
Good luck!