I worked for a DOD Contractor that did this for about 500+ users for a few years. What you save in upfront per-machine cost, you will pay back with extra in manpower and frustrated users. Keep the Dell Contract, negotiate for discounts based on your size and the fact that you "have a workable alternative"
Think of this, 1000+ Machines will need in-house IT to manage. You WILL have at least 5 on the bench at any given time. Just accept that. You will have to employ someone at each site to manage those machines, staff who will be idle at some of that time.
With vendor PCs and Onsite contracts, you will STILL have 5 on the Bench. But it's not YOUR Bench and the user community will join you in commiserating about the vendor rather than complain about you, at the same time, you will NOT be paying for staff to manage all those machines, you will simply call the vendor and have them come to the site as per contract.
No muss no fuss.
As a tech, 1000+ hand built machines with full control was absolutely awesome!
From a management standpoint, it doesn't make sense.
Growing up sucks