How can I gather intel on a small private company?

There are marketing intelligence companies that you can hire.

What do you need to know? Have you clearly defined your requirements by writing them down?

I assume you know what industry they are in?

Start with their Website?
What service or product do they claim to offer?
Do they mention who their clients are?
Where are they located? 1 city, 1 state, nationally?
What does their HQ look like ? Standalone building? 1 floor in a larger building? 1 office on one floor in a building?
How many people do you think can they fit in their HQ? Is there enough room for just Management or do they have space for larger operations?
Who are their direct competitors, how many are there?, how much of the market share do the claim to have? What is the market share worth? What is the market like?
Are they mentioned in New Artciles, Industry publications and etc?
What tools do they need to do their job?

Why don't you visit their HQ? Get an associate to get in touch with their Sales team and setup a meeting where they do a sales pitch on you as a perspective client for them? This doesen't mean you approach them with your real intentions. Just say you are such and such and thought they might have the capability to provide you with whatever service/product they provide and were interested in having a chat with them about it. Sales people have a wonderful knack of usually telling you more than they should.

You should be able to figure out most of the above before needing to engage an external agency.

One thing you will not get in black and white (unless you decide to break in, bribe a company director, bribe their bank manager and etc) is an exact P&L nor a written business strategy plan (assuming they have one).

Buy a Dun and Bradstreet report on them. D&B has a record on pretty much every company in the US, with varying degrees of accuracy. It will give a lot of the detail your looking for (potentially as much as complete financial statements). Hoovers actually gets a lot of their data (like the revenue # you got) from D&B.

Um, step 1 is usually to pull a Dun and Bradstreet (or other equivalent) report. D&B covers the US market pretty well. D&B codes businesses by size and there's a whole rating code system that's pretty easy to understand and will start to give you the basics.

Assuming they're incorporated, you'll find general info on principals, ages of principals, amount of time in business, and often some other financial info, if they've disclosed it. Even if they haven't, often there will be info on credit lines etc. they have open which may provide some further insight on size, creditworthiness, viability, etc.

Edit: beaten to the punch by 2 mins.

Usually it's total bullshit - they estimate and sometimes call/mail the business to confirm information, but if they don't get confirmation, i think they just go ahead and publish the estimate. As a former business owner, I remember getting those types of calls.

An undercover operation is the only accurate method.

But they'll report it as an 'estimate' - which should be taken with a grain of salt. Sometimes they do actually have full financials. They're pretty good at telling you what is fact and what is an estimate.

In your spare time I would also suggest you flick through Michael Porters classic Competitive Strategy: Techniques for Analyzing Industries and Competitors (Look it up on Amazon.)

He walks you through how you go about doing an Industry and Competitor Analysis and has checklists. The latter is in principle pretty much what you are doing by trying to find stuff out about your target company eventhough you have a different purpose.

Industrial espionage is illegal, thus I can't recommend it.

Do you have a budget on this?