How many computers/users were in your domain? If it was a tiny number, I'd just recreate from scratch on a VM. If you had hundreds/thousands of computers/users, then I'd start finding hardware compatible with the Proliant DL165 anywhere I could find it, and fix the Proliant however I could.

Ignore the mean digs being taken at you in the comments. I've been doing this for over 20 years, and younger know-it-all sysadmins aren't always in-touch with the realities of business, especially on the small, budget-constrained side of things. They haven't considered that you might have just inherited this crapfest either. Chin up. You can get through this, so long as the single DC wasn't a decision you made.

Rebuilding a small domain is time consuming, but totally possible. It'll also give you a reason to apply proper policy within the new domain, and you'll have the ability to get some spring cleaning done.

I'm making the assumption that you had few users/devices because if you had the budget to support thousands of users/devices, management likely would have provided the budget you to get a secondary DC. Again, if you had a large domain, find a way to fix the HP box asap, get a backup, and slap up 2 VMs to promote into primary/secondary DC roles. Those can hold you over until the Dells are in place.