I feel building a project from scratch with nothing in a weekend is no hugely likely. Most people I figure have all there ducks lined up so they are ready to hit the ground running 6pm Friday night.
An idea, maybe a rough wireframe all IDE's and tools ready to go.
I would go through the numbers with a fine tooth comb
For the number of accounts find out the number of actual active paying accounts if applicable. Some other numbers if they have them like advertising dollars spent per customer gained would give you an idea of the business viability.
The main thing you will get out of buying this is customers. I would also check though whether you are just getting a reseller account or are you getting rack space along with some servers that the customers are hosted on.
Web hosting companies usually come with some history. Do a search for the company on sites like www.webhostingtalk.com to see what people are saying.
Also I would ask them about their payment gateway and there current standing with their payment processor. Do they have 1000's of charge backs and are in poor standing or do they have measures in place to prevent this and all is rosy with the processor.
From a legal point of view get someone (depending on local laws) to trawl though all there accounts to make sure there is nothing illegal i.e. pornography, file sharing etc as you don't want to be buying into something that could get you into serious trouble. Also make sure they have legit software licenses for any non free software that is in use.
As staunch mentioned getting an accountant to go through their books is a no brainer.
I hope this helps it was a bit of a brain dump. I can go into more info about different topics if need be.
An idea, maybe a rough wireframe all IDE's and tools ready to go.