Hello there @crypto.piotr it is in deed a great initiative. Thanks for sharing it.

How would you structure reward system having a 300 steem budget?

Is a good question. I think the approach of giving between 5 - 10 prizes is good. I know from some my experiences that always it will be some people discouraged but this is normal. The thing is that everyone should feel part of it. So maybe:

1. If you partner up with 1 or 2 curation communities you may give all the rest of competitors at least a vote so they may win some reputation, exposure and some Steem Power. This is what I think to do it more fair and also to motivate people within the contest.
2. If you find a nice whale who may support you guys as well, at least for any contest, you can give more value in return.

Future Tip: If the contests grow enough you may consider to ask for a small amount of SBD or STEEM to each participant in order to power up(temporarly using @Blockctrades Sp delegation rent) then you may give more value.

_{I have a curation community but we only curate Spanish content, so if you plan to keep it up with the contests and also adventure into bilingual posts(ENG/ESP) I could you support you guys as well. We do have no more than 2,000 SP but we have a small trail so we may support a bit.}

STEEM ON.