I'm currently putting together a lightweight commuter / flat bar roadbike. Granted, the carbon FF was in the parts bin, and lots of the drivetrain is coming off a wrecked and dumped MTB thing that a neighbour hauled out the bin for me, but it's going to come in at a lot less than $1,300. The frame is a recently powdercoated road frame I got for €25 on local pickup eBay.
It should be around €80 all in if it all comes together OK and I will have a fully functional +/- 9kg commuter bike.
Point being it's much more fun not to throw money at a project like this - at least for me.

Now, on topic, rather than panniers, I got a KlickFix rucksack with a saddle mount, and it is absolutely bloody marvellous, I can't recommend it highly enough really. Might be an option for you as you have a longish walk from bike park to office.