OK, so the OP asked about 1000 desktops, but I thought I'd say something anyway. Our server team decided to do this. It started out with a few in-house-built custom servers because we couldn't get what we wanted from Dell. We settled with Supermicro as our MB supplier as they hands-down have the best selection of server/workstation MB's, and are much quicker at including newer tech that the likes of HP, Dell, IBM, etc. As it came time to replace more and more servers I identified a vendor that would build our machines with the parts we wanted burn them in. We kept a handle on the different types of motherboards we used as we were stocking spare parts, and had other support benefits. Plus, if we needed something from Supermicro or our server vendor they both were only 30-40 minutes away. I want to see Dell or HP provide that ;-). It was great.
Another benefit came from the fact that we worked in an area where classified work went on, and not having to escort Dell techs was also a plus. Of course we had actually stopped doing this years ago because it was almost impossible to get someone into the classified area in four hours, so we ended up changing our Dell purchases to four-hour parts only support; but I digress.
We also could consistently beat Dell pricing even for our Windows servers (due to an M$ enterprise agreement the lab had). Dell charges an arm and a leg for memory and hard drives, so this was generally where the big savings came from.
OK. The point is that what we did could be scaled up easily. All it takes is a team that knows what they are doing, doesn't go config-crazy and use every motherboard under the sun, etc. Identify what your users do and what their needs are, create your configs from this trying to keep as few as possible, create images as a vendor won't want to deal with install scripts, buy some spare parts, and you're done. Shampoo, rinse, repeat at whatever interval meets the requirements of your environment.
We actually tried to do this with our Linux desktops as our server team handled all the Linux support short of actual desktop/user-facing support. We got shut down because of the byzantine rule that all non-Apple desktops and laptops had to be Dell.