- you could invest some BTC for development bounties to improve the code/protocol of Bitcoin
- and some BTC to buy more equipment
The simplest rule would be to use all funds for mining (electricity + new equipment). I obviously have no idea where that eventually would put an organization like this in terms of hash power. If created as a foundation I guess its purpose could be specified somewhat vague (to allow for some later decisions on how to use the funds) but not too vague. Perhaps something along the lines of "contributing to the security of the Bitcoin network".
I think that solo mining would be fine since variance doesn't really matter that much.