If you're thinking of getting the new hardware, replacing the old physical server, to run ESXi 4.x, then you can do it. I would actually plan on building the host while the old box is still online, make a new domain controller VM on the host, then retire the old domain controller once it's online. I would also suggest at least building up the VM's as single duty entities. Such as have the email server as one VM, SQL run on another, etc. If you're going to use just local storage on the host, then get at least 10k RPM SAS drives. 15k RPM drives will be better, with a solid RAID configuration (and controller). I would also opt to get enough Gb NIC's inside the host from the start. For a small deployment, using local SAS drives, you could probably get away with 4-6 ports. You can always add more later as you need. But, this way you'll be able to build in some redundancy, and account for bandwidth needs (expected now, with room to flex a bit).
I have a home test lab setup with a single ESXi host server (dual E5405 Xeon's, 16GB RAM, two 146GB 15k RPM SAS drives where ESXi resides) with a NAS/SAN (QNAP 5 drive bay unit, populated with 1TB drives), HP ProCurve 2510G-24 switch. I'm running 7 VM's currently (was 8, but I'm not running the vMA right now, I'll spin that back up later). Included in that list of seven VM's is a vCenter Server, two SQL 2005 VM's (one 32 bit the other 64 bit), an Exchange 2010 server, my AD DC box (does DNS and DHCP too), a linux FTP server and a linux web server. I'm running at 10-11GB of RAM utilized these days. I am planning on adding more soon, so that I'll be able to spin up more VM's on the host. Memory is one area where you'll quickly run low. Storage is another. Probably faster than you think. I'm good in the storage space right now, but I need more RAM so that I can run more VM's, such as a vDR, vMA (again) and whatever else strikes my fancy.
You could probably get a tower server to start with. Install ESXi onto a flash drive, and use SAS drives for the VM's. Just be aware of the strength's/weaknesses of the different RAID levels before you settle on one. RAID 5 looks nice at first glance, but you'll need enough spindles to offset the performance hit. Just make sure you don't max out the box from the start. Well, unless you go to way over the top levels...
