Use disc fork on an Lynskey R260? I got a great deal for a Lynskey R260, that is set for caliper brakes. I'm building my first bike from scratch and am thinking of using disc brakes on the front fork. Is there any reason why I wouldn't be able to use this fork with the R260?

I understand I'll need to use caliper brakes on the rear. Is it possible to mix and match?
Yes, you can use that fork and mixmatch components. Of course you'd have to check parts compatibility, here's what to look for:


Stock and replacement fork rake/axle to crown measurements. Pro Carbon Road #2, which is offered as a stock option for that frame, and Pro Carbon Road #5, which you want, both have rake of 45mm and axle to crown of 368mm, so everything's good.
Then make sure the frame accepts you fork of choice steerer type (i.e. 1-1/8 to 1.5 Inch Tapered). Again, these match, so good here too.
Fork crown shape. Totally not the issue in your case, but some frames/forks have weird crown shapes that pretty much lock you into specific vendor options.


The rest comes down to components choice:


What brake levers are you going to run? Options are: a) full mechanical, b) one mechanical and one hydraulic (but the shape might be too different) and c) full hydraulic (there are hydro rim calipers, but make sure those will work with the frame).
You'd have to build a custom wheelset or purchase front/rear wheels separately. Rear QR and thru-axle in the front is no problem at all, I run the same and some bikes come with such a setup out of factory.