Lots of heartache and pain.
You're looking at short term savings to inflate long term expenses.
Sure, it seems like it'll save you money in the beginning, but then components start to fail, and you can't find exact replacements. Then components that you bought to work with the old system don't work with the new motherboard/videocard/something. Then you think "we'll have two disk images". Then, a few months down the road, something else becomes hard to buy. Three disk images.... and so on. And you have to remember the quirks of each system as you set work with them.
Assuming you don't buy 1000 groups of parts immediately. (You won't, no accountant out there will approve it. Basic business needs say that's a bad management of cash flow.), You'll be spreading that purchase over 6 months or a year. I don't know if you've noticed how often basic components get refreshed, but by the end of the year, you won't have 1000 identical PCs.
I know calling support for Dell is a pain in the butt, but try calling tech support for Asus. In Taiwan. During business hours EDT. Plus, you can't expect them to keep spares for the time you need them, where most OEMs keep spares on the shelf for 3 years. Do you have the warehouse space for that? What if there's another run of bad capacitors, and all components manufactured during a time period are bad.
And... then there's your time. At my peak, I could assemble and build a PC in about 3 hours. Multiply that by 1000. That's 3000 hours. Non-stop building. There are only 2080 working hours in a year. When are you going to have time to do your actual job, that of system administration? Yes, you can hire someone, but so you want to hire a person who assembles PCs, or do you want to another system admin, which will actually make your job easier?
We're not even talking about the government's needs to track where money is spent. How are you going to stick asset tags on a random bunch of assembled components? What happens when most of the guts of a PC get put into a case where it already has an existing asset tag?
Man... I've beendown this road. We got about 18 months down it and we went back to the OEMs. Dell, HP, etc.
If you're going down to the local PC store, or buying direct from Dell's retail side, STOP. Look into Dell's corporate line of PCs and the HP's corporate line. I just checked HP's government purchasing site, and you can get a small form factor PC for about $350. I'm sure RAM isn't that expensive, and the three year service contract in bulk won't be that bad. If you're in the state of Virginia (and if you're working for the government, there's a high likelihood that you are), consult an eVA price list. Or go off the VITA contract. The amount of PC you can get for very little money at government pricing is somewhat ridiculous.
Just, please, no, don't go down the path you're going. If you really really really want to, do it in one department only, for 18 months. See if it's worth the hassle. I'm betting it's not.
These thoughts are all just dashed off. I'm willing to go into more specific detail if needed. I just remember how happy we were to get out last HP Vectra machine in to replace the custom built pieces of crap we had before. It was nice in so many ways. I could actually go back to administration and not construction.