Having some sort of BMC is a must for an ESXi server in my opinion. You can definitely roll your own with a SuperMicro server mobo, but there are also cheap PowerEdge tower servers on Ebay.
What's your budget like?
I agree with mfenn on the BMC (or an IP KVM). You can setup your lab with enterprise grade equipment on the cheap as long as you don't "need" the latest equipment. I don't have anything against building your own, I'm running 2 Supermicro systems myself right now. But HP Proliant G5's, Poweredge 1950/2650's, and IBM xServer M1's are all very affordable on eBay/Craigslist. Far cheaper than building your own. Shameless plug (sorry if that's not allowed), I've got an IBM x3650 M1 for sale right now with a pair of X5355 Quad Cores and 20Gb of RAM for $300 + shipping.
Here's my thoughts on building home labs with enterprise level gear on the cheap (meaning sub-$1,000 in my book). You're pretty much looking at Xeon 5000 series which "generation" depends on how cheap you want to go. Xeon 50xx - 54xx use DDR2, max 4 Cores, No HT. Xeon 55xx - 56xx use DDR3, max 6 cores, and are available with HT. They are also more efficient meaning they use less power and generate less heat. Performance wise, realistically a pair of 5300/5400 quads is probably more than enough for a home lab. Memory prices are pretty even between them right now, but the DDR2 FB-DIMM's run VERY hot for memory. Even with enterprise level gear you can run into cooling issues.
I recommend sticking with HP when looking at this generation of equipment. For the "first" generation Xeon 5000's, the Poweredge and xSeries don't support the 5400 series Xeon's. As far as it working even though it's not supported, I don't have any experience with those Poweredges, but I can tell you on the xSeries only 1 revision of board supports the 5400 series Xeon's and the system part number is the same so the only way you'll know is by booting into the BIOS and checking the board revision. You're typically going to be looking at $200-$300 plus shipping for this generation.
For the second generation, it's a really close match between HP and Dell (IBM's carry too much of a premium). Typically you'd be looking at either a DL360/380 G5 or Poweredge R610/710. You'll be looking closer to $500-$600 plus shipping for this generation.
HP Pro's: They include their BMC with a dedicated NIC port, it's optional on the Poweredge (shares a NIC port by default). The HP's have their power supplies on the left when looking at the back, like every other piece of hardware I own, whereas the Dell's are on the right. Just an annoyance when doing cable management. HP's internal setup is a bit more refined (IE fans with integrated plugs rather than cables).
Dell Pro's: Slightly cheaper, slightly quieter. I really do like the little LCD on the front, I set it to show power usage on my home server.
If you're set on going with a tower, that limits the options significantly. As somebody who's got tower model Poweredge's and xSeries, I'd stick with rackmount, even if you just stack them on the floor. The towers are too deep to fit under a desk anyways, so why bother.
I've actually been meaning to make a budget server lab thread for a while. mfenn, would you be opposed to me doing that and stickying it? We get people asking about cheap server builds pretty regularly.
