You ought to hire a consultant, sounds like.
If you have money to license IP, you should have money for a consultant to evaluate the value of the game.
Note, I'm not suggesting myself. This is a bit outside my area. Essentially, you need to write a business plan.

I agree with Tom. Get some professional help from an IP lawyer or consultant with experience in pick-up or asset purchase deals to help you do some diligence on the underlying rights in the game: (1) Did the seller secure all work for hire agreements with contractors? (2) Can the seller assign rights to middleware tech incorporated in the game? (3) Did the seller create all in-game assets or are some licensed from third parties (e.g. music, models etc.)? (4)Did the seller file for any trademark or domain name rights?

This is only a starting point to give you an understanding of what to base your offer. If you have to go out and obtain other rights to launch the game, then the value of the seller's IP is reduced.

You will also need to evaluate the code to see that this is actually 90% complete - then you need to work out what it will cost to finish the game and get it to market.