If you're doing it for financial gain, consider - why is it up for sale? Very few people would go to the trouble of building a profitable game, then selling it - unless they thought they could get more money from the sale than the game, or they needed some fast cash.
Get it clear and in writing, what exactly is included in the sale and get administration documentation before you purchase - know exactly what will be involved in running the game - then you can decide how much of your time it will take, how many mods you will need and whether its worth the hassle.
Owning a popular online game sounds fun but if it takes all your free time in admin tasks and bug fixes then the novelty will soon wear off.
Dont rely on free help from friends - if its available, thats great but dont build a business plan around it. Just because they're your friends, doesnt mean they're obligated to give you free help on this project - RL happens - you cant rely on anyone who works for free - no matter how close you are.
Also, if you are doing this for profit - ask to see at least the last 3 months accounts - 12 if they've got them. How much income the game makes every month will determine how much you can spend on staff and improving the game.