What type of sauce is appropriate for a dry-aged prime steak?
This sounds yummy!  

I'd recommend cooking sous vide (SV) as you suggest (130 F for 1-3 hrs). I tend not to season in the SV bag at all because salt &amp; spices often turn out oddly; for example, raw herbs/onion/garlic are far stronger and taste "uncooked". Figuring out how to spice SV takes a fair amount of experimentation and is best done with dried &amp; powdered herbs/spices instead of fresh ones. 

VERY IMPORTANT:


Before cooking SV trim excess fat from the steak
After cooking take the meat from the SV bag and DRY it thoroughly before searing! 
If you're planning to use bag juices, first bring them to a full boil in another pot and skim off the scum (myoglobin "blood"), then use whatever is left as your meat juice.


To sear, pre-heat the cast iron sear to a very high temperature (~500) and use a high-temperature oil like peanut.  

The sear should be at VERY high heat for only 30 seconds or so per side - if you leave it on much longer you'll lose the beautiful 130 degree edge-to-edge miracle that should be your reward.  

Using butter for the sear is not a good idea as either the butter will be destroyed by the properly high heat OR the steak will be destroyed by keeping the cast iron's heat low enough for the butter.  Rather, use compound butter as a topper for presentation.

As for using a torch, I've used a brulee torch a few times and the heat &amp; coverage is insufficient to sear a steak well with my limited experience. You can sign your name on the steak with it, but getting a good, even edge-to-edge Maillard crust is very tough.  Using a MAPP torch is quite a bit better; MAPP gas is flavorless, it's hotter than propane or butane, and MAPP torches tend to fan out broader flames, but I still don't think they create as dense or flavorful a crust as a pan-sear. Moreover ANY torch runs a likelihood of flat-out burning areas of the meat, and that's especially true for a first-timer!

Let's talk about salt before going any further. I've always salted meat before grilling. But SV is a completely different ballgame. Salting before SV doesn't add much flavor for you on short cooks and hurts badly on longer ones (cures the meat). There are online arguments about this, but I prefer salting only after SV and preferably after searing. Taste tests support this post-SV salting approach - check out http://www.cookingissues.com/2011/10/12/to-salt-or-not-to-salt-%E2%80%93that%E2%80%99s-the-searing-question/ for a great take on the subject.

If you keep the searing time appropriately short you won't have to rest the meat before serving.  The meat is still incredibly juicy and doesn't need time to reabsorb fluids.

As for what to do with a sauce, I'd go for the butter instead and incorporate a steak-friendly sauce into a side dish. 

A quality hunk of well-marbled dry-aged beef should sing with this preparation and I'd put it center-stage to sing solo, with the side dish's sauce grooving nearby ready to join in on the chorus.