First choice would be the Cab Forward. Last of it's once populace breed and utterly iconic of railroading in the western US.
However CSRM has it stuffed and mounted so forget about it. Same for NPC #12 "Sonoma", the museum likes to talk about getting it running, but it'll never happen. They'd funnel the money into advertising.
Pretty much the same could be said about the "Jupiter" at the Smithsonian.
NCNG #5 would be a blast to see running again, but it's land locked and is the center piece of the Nevada County museum. It would be runnable but never run.
The best choice would be "Glenbrook" at NSRM. They've already done most of the work, have a tender ready to go. Just need a proper boiler and the money to put her back together. She'd be a great match with the "Eureka" and is owned by a museum that prides itself on having well maintained and operational equipment. It would take less than half of the million to get her running and the remainder can go into restoring the Lake Tahoe Railway combine for her to pull.
Now if I wanted something for myself there would be a few criteria.
1) Must be privately owned, as Museums and Municipalities are unlikely to sell.
2) Must be in fair condition as it's only a million dollars.
3) Must have a place to run it / store it.
I'd love to get my hands on the Sierra Railroad #18, or #34 or #36. However the current owner is not known for being reasonable, and where could I run a standard gauge steam engine?
Next on my list would be Death Valley RR #2. A big 1916 inside framed 2-8-0, simple, elegant, privately owned, and stored in a desert. Saw her just last week and her condition looks excellent. I'd love to owner her. Then take her up to Sumpter Valley for restoration and then some running.
Ok, that's enough rambling.
Cheers,
Curtis F.