I was thinking about 15-20K.
i really like the coingaming.io platform. no idea how much it costs
If I were you, I'm just going to hold it and keep on saving bitcoins and wait until the right time comes to convert it. It is too dangerous if you are going to make a gambling site with partners and you are not knowledgeable about it, I don't think that it is a good choice. You will easily be fooled and get scam by your partners if you don't know all the details about your gambling site, if you are really going to make it.
And 20K is not enough on building one, many players would gamble on your site of course, so, you should set the maximum amount of money that your team are going to be estimated, and I'm sure that it is not just 20K USD, it is between, half a million or so.