Some of the paint has been stripped off. Is this worse than just cosmetic? Will bare alu corrode? Can I just clean it up and spray it myself?

Cosmetics are not a consideration since the Prius has hubcaps/wheel trim on top of the alloys.

Will bare alu corrode

Yes but a regular polish will keeping it looking OK

Sound like an engine/seats/dash out job to me. Chop chop, get to it.

I could perhaps just polish it up and spray it with any old paint?

http://www.halfords.com/motoring/paints-body-repairs/specialist-decorative-paints/halfords-heavy-duty-pu-lacquer-500ml

Of if it’s just a bit in a non-visible area then seal with some clear nail varnish if you are really worried.

Should do it if you can make a decent job

Any corrosion won’t be structural just a bit rough to look at

I’d be more concerned about how the brake fluid got there. About £50 to strip and repaint a wheel (properly), may be worth while doing.

It got there because the wheel was handy when I was using the Hope bleed kit to bleed the brakes on my bike 🙂

You need to make a hot poultice from flat irn bru, fullers earth and Jam from a hipsters jar !

Spread liberally and rub like a mad man.

The alloy shouldn’t corrode any more provided the brake fluid is all removed. You can get wheels touched up for sensible money – or get the whole wheel resprayed for £50 (as said above)