So I'm going to just start off by saying that there is no right answer. Specially because we don't know your needs, we don't know your environment, and, specially because this is your first try at elastix, we don't know skills/ability with asterisk or telephony in general. Now that that's out of the way, let's get down to business.
I originally come from a world of Cisco, Avaya, 3COM, and Nortel. From a quality of build perspective, and also from what I've been exposed to, I like the Cisco phones. With elastix, except for a minor edit to the MAC table, I found that the Cisco 7960G phones were quite easy to setup via the Endpoint manager. I was also able to easily find the default configuration sheet for the sip default and sip(mac) files and easily enabled options there that further enhanced the configs I made with the Endpoint Manager. Now, if you're not comfortable with editing some files for automatic provisionning, and you would rather manually setup each phone seperately, then Cisco isn't for you. You're better off with Aastra, Grandstream, Polycom, etc... I guess the Yealink is a fast up and comer as well. I know all of this is pretty generalized, but the considerations and options are pretty huge. So my 2 cents, basically what works for me right now, is an old IBM x330 server with a 3ware raid controller and a pair of SATA 7200RPM drives in a RAID 1 along with some Cisco 7960G is what I'm currently testing. I'm sure others will have vast number of other combinations they would recommend as there is no "best" answer.
As far as the elastix server itself, as mentioned above, it's running on an old IBM x330 server with a mirrored set of hard drives. The 1U pizza box server is annoyingly loud, so don't set one up next to your users, they'll hait you for it. LOL. Anyway, there are those who have used some Atom based PCs/motherboards to get equal success for a smaller userbase. For the most part, specially if you keep your environment on the same codec end to end (no transcoding), then your processing power and memory requirements do not have to be huge.
My best starter suggestion would be this. If this is your first attempt at Elastix, specially if this is your first attempt at Asterisk in general, then start with an old computer, maybe an old P4 with atleast 512mb ram, install it and play with elastix. Forget about the phones for now and use a softphone like x-lite. Get yourself comfortable with Elastix before you start diving into phone decisions.
I hope this was helpful. =0) Happy telephony.
Tried AsteriskNOW - You'd think Digium would have the superior product, but they went for the stuperior product
Tried TrixBox - They got all corporate and forgot about the rest of us
Tried PIAF - Took a golf club to it
Elastix ??? LOVING IT ???