Just wondering what the differences are between an Interarms MK-X and a Husqvarna 640 action. I understand they are both commercial M98 design and most of the 640’s are not D&T and are of an earlier vintage than the Mk-X.

Are there any obvious choices of one or the other for the basis of a custom rifle?

Easy choice, and a ton of differences; most significantly, the country and years of manufactures. All Husky 98s are made at FN; Sweden never made a 98.
The MKX were made in Yugoslavia and are not as well finished.
Buy the FN/Husqvarna EVERY TIME.
True, they were not factory drilled.
Oh, nothing wrong with the MKX; I have used a lot of them, and in fact started in 1970 building rifles on Herters J9s, which was actually a MKX.

and I'd go the other way -- if faced with this choice, I'd go MkX IF it has the in the trigger bow floorplate release -

Ah; I wasn't thinking about that; if you want the hinged floor plate instead of the military one, yes, the MKX has that feature.
I pay no attention to such things, using military bottom metal for my own rifles.

Husqvarna 46 is a Swede produced 96
Husqvarna 146 is a Swede built 98 on a pre war FN action.

Husqvarna 640 is still a "true" 98 action, FN produced, what some refer to as a "double broach" which is a misnomer, this action is a FN "Supreme" action, though there are lots of options and variations. Some have hinged bottom metal and some do not.

Stay away from the 1640's in my opinion. They were the last version and not a true 98.

I would, do, and many times have, chosen a 146 over anything else. Machining is FAR better than a Mark X. Fit and finish is far better than a Mark X. Slap a 1909 bottom metal on it and you have a fantastic action.