Proofreading a 90k words thesis is hard work. It will take you at least 3-4 days (probably more).

Most professional proofreaders charge well over 500 euro for such a job. According to a price calculator I've found you could charge 735 euro. I think that is still cheap.

I've used this price calculator: https://www.scribbr.com/proofreading-editing/rates/

You can use a working paper. However you should give it way less credit than a published paper. Publishing a paper usually takes years (doing research, writing draft paper, contact journals, get it peer reviewed etc. etc.). A working paper is usually a good draft.

Use it in this way: "Currently Johnson is researching x and his findings contradict y. This thesis will use the same methods as Johnson...". So just use it for "inspiration" and it is probably better to not use it for facts. This way you will be safe ;-)