Offering you one way of investing the money is ignoring the importance of diversification...so I would invest in three sound investments...if going by what Josima said is true...then using 300k to buy 1/8 at Isinya would work. 150k in bonds and 150k for an m-pesa outlet and employ someone.Just make sure the till is in a populated area like a bus terminus. Avoid stocks for now and do thorough study when buying bonds.
A successful man is not he who gets the best, it is he who makes the best from what he gets.