Hi folks,
Aside from cost, is there any reason to not add disc brake tabs to my old Trek 750 1" threaded fork?
Alternatively is there anyplace I can just get a new 1" fork with disc tabs, & preferable front rack mounts if that's available as well. Threaded preferred, but I could figure out threadless I spose

Thanks for any thoughts ya may have on this

I would be wary about adding disc brake tabs to a fork which was not meant to handle the increased leverage and power a disc brake adds.

As far as 1" threaded forks with disc tabs, it's pretty impossible to find them.

I would get a custom shop to braze/weld you a pair. IRD might do one, as might Tange. The same strenght problem would apply to your frame seat stays though.
Considered buying a Poprad frameset?

Disc brakes load the fork very differently from caliper brakes and a fork not originally designed for them is a poor candidate for their addition. As noted, fitting a rear disc mount to a frame not spec'ed for them is also problematic.

Hadn't considered the seat stay thought; although I'm inclined to think they're fairly sturdy. I hadn't considered the Poprad either; however I'm not sure if it will fit 35mm's, & fenders. Considering Trek I'd be willing to guess not. I'd thought 'bout their commuter but saw it didn't look like it would fit full fenders either..You'd think something called "Portland" would have that.
There's a local shop 'round here "Ticycles" who would do the braising for 'round 250ish per their site, 'course I'd hafta repaint the sections, but since the bike's gloss black I think it would be fairly easy to match it. 'tis a thought since the weather round here's a bit dank in the winter, spring, & fall..& of course the summer sometimes as well.

Tange's site didnt' go into detail on their forks, & IRD's all 1.125. alas no easy way out of it.

Thanks folks, Please post any further thoughts, I ain't taken the plunge, but still considering it.

Check these forks:

https://www.viciouscycles.com/forks.php3

Fitting the disc tabs to the frame seat stays might not be the best idea though - for the reasons already mentioned.