Thanks for the stats quote, while I like how the number is symmetric, I have to keep going forward, maybe somewhere in the future I'll get a longer symmetrical number 

As for the issues you addressed:
1- We're building the game together using free resources, Unity offers free license as long as we don't have 100k$ yearly, and there are lots of free assets around, so it's actually just investing the break time at university.
2- Trading will be a side thing for me. I'm not abandoning my game idea, just not being 100% dedicated to it, because as with every project it could fail at any point in the future, so it's always good idea to have alternatives later.

How possible for the website to be scam? I guess it's very likely. Why? simple, they don't provide enough info about who they are, not a registered company, and most importantly if you visit their FAQ
You know there are tons of ways to hijack such 'cheap' method to pick a winner, it would have been totally different if they provided a provably fair method.

The whole thing seems sketchy to me, because I've tried to make a provably fair lottery before, just failed with marketting, and it didn't need any registeration or user information too.

Don't take my advice without extra thinking, I'm mere saying what I think I'd do if I were you:

2. Do i need to buy bitcoin from somewhere else and deposit in the wallet address?
I guess you'll need to have a starting capital in all currencies that you'd like to add in your exchange, it'll allow you to initiate a faster flow of users, maybe you should look for users that sell/buy regularly and ask them to use your platform to kick start your trading volume.

4. About about buying and selling price? what's your suggestion?
I suggest you check the existing platforms and see their rates, then see your current and expected expenses, and from there you can start placing a range for exchange.

5. Timeline would be important between payment deposit  and assigning them bitcoins, how to reduce to minimum?
Do you mean confirmation time? if so, with segwit it should start getting lower, I believe all exchanges require minimum of 3 confirmations before crediting your account. However, if you have high volume and prepared to take small loses, you can credit deposits after 0~1 confirmation, just make sure to lock the user from withdrawing funds until the transaction confirms.

6. Plan to charge only 5-7% of transnational amount, any calculation on profit?
Guess you'll have to do it your self, hire a proffisional to do it, or wait until someone with more experience to answer it.

7. Anything else you would like to suggest?
Make sure you have legal matters covered, so make sure to discuss it with a lawyer depending on your location and your company location. Also, soon as you start rolling some profits, make a solid investment in security, because getting hacked will cost you lots. Finally, provide proper training for whoever works with you, and make sure they don't leak secrets, data, logins, or whatever, and try not to trust based on humanity, make sure everything is signed and documented.

Hope you find this useful.

Me and my team are currently developing a game, and started looking into the monetization part, just the usual ideas going around are banner ads, shop items, and video rewards.
However, we're a group of crypto lovers (been trying to get into the crypto world for a year or so, but funds are always a problem) -creating the game it self was a method we thought of to get some funding to start trading-. So, we had an idea, instead of implementing the normal ads into the game then using the money to buy crypto, wouldn't it be better if there's an ad network/platform that already support paying out in crypto?
The game currently under development using Unity game engine, so the best case scenario would be using an existing plugin to integrate. However if none exist, we could implement it our way.

Is there such a solution? and in your opinion, would it be better than normal ad networks or not??