Bush is one of the products under his 'Biggest Brightest Games' category. So you'd need contracts drawn up to mean he won't be able to use the source codes or create a similar product to rival you if you see fit.
The figures are totally unknown but I believe the bulk of his income used to come from Game Cash.
You also need to factor in as a purchaser your loss of earnings from alternative means, as coding and maintaining the game, if you wish to make it successful enough to live on will take a lot of time.
The biggest issue is the name of the game, that's the main thing which has caused me to not inform many of my friends I play "Hey mate, this game called Bushtarion kicks ass, you have to attack with Gardeners and steal other people's acres" - just no.
I think the code could be used effectivly in other genres but unless drastic changes occur, then you won't be able to make much money.
Where did you get £20k from...?