Enterprise Server

If you're buying one for your home lab, it's most likely going to be used.  Even at the price of a used one off of eBay, they stretch the definition of viable for a home lab.

There are only two cases where I'd really bother to recommend one and that's if you're trying to learn for the sake of a certification (such as the various Cisco certs) or if you've got money to burn and just want to do it for the giggles.

There's really not much I can say for a buying guide, aside from saying to do your own research on which models will actually serve your needs and that you'll probably find your best deals on eBay.  If you're lucky, you might be able to scavenge some for yourself at a business/school that is either closing down permanently or in the middle of a hardware refresh.  However, know that you'll likely be competing with professional resellers to grab any.  This comes down to connections and luck.

Some particular caveats worth mentioning are:

Power Hungry - These things are designed for 100% uptime and the cost of electricity rarely enters the equation for their design.

Noise - These are typically shoved into closets where they can emulate jet engine noises all they want.

Proprietary - Businesses that make these often come up with contrived hardware DRM just to charge extra.  They get away with it because the businesses that buy this stuff are used to paying in the order of 5 to 6 figures for even just one server.  Don't be surprised if the server you buy has some kind of hardware gotcha hidden inside that dramatically ups the cost of running it.  Some of these can be circumvented cheaply and some can't.

Compatibility - Related to the previously mentioned propriety drawback.  Due to the proprietary nature of these servers, they are often severely limited in what kind of hardware works with them.