Yeah, I wouldn’t think Google would penalize you for having 20,000 in pages go live all at once. They are mostly concerned with “unique” content, so as long as you aren’t throwing up 20,000 pages that all have one keyword changed to “game” the search engines, you would be fine. You should rank fine for non-competitive terms (if there are any in gambling), but only lots of time and plenty of backlinks give you the competitive rankings.
I agree with Dom, if you have £1M to invest, spend the time to search out one of the better performing affiliate sites and invest like you would for any sort of venture…on a valuation of the business.