It depends how long you're going to keep it in secondary, since you'll get some oxygen ingress with a bottling bucket. Should be fine for a month or so, but any longer than that and I would be a bit worried. Typical brewing sanitizers will kill brett, but I think its good to be cautious since a small number of cells can completely change a batch of beer. That said, I share some better bottles between funky beers and other non-funky but highly attenuated beers without any noticeable contamination so far.
If you can get fresh bottle dregs, I don't see any advantage in getting a commercial strain (maybe others disagree?). But if the bottles you have access to are likely to have been sitting on the shelves for months, it might be worth getting a proper pitch.
A rough guide for priming is 2 volumes of CO2 for each gravity point in the beer. Depending on your OG, there is a good chance that the brett will eventually take your beer down to 1.000 or lower. So if you were bottling at 1.004, you should assume that you'll get 2 extra volumes C02 as the brett takes the beer down to 1.000 in the bottle, and you should take that into account when calculating your priming sugar: say you wanted 3.5 volumes, you could add enough sugar for 1.5 volumes and then wait for the brett to do the rest.
That said, the nice thing about 3711 is that in my experience it reliably takes beers all the way down to ~1.000, so there shouldn't be too much extra C02. (If you plan on keeping bottles around for a while its good to keep an eye on carbonation levels by drinking one occasionally: I read something by Chad Yakobson somewhere that said that sacch cells can release sugars as they decompose, so you might eventually get a bump in C02 if you keep the beer around long enough).
Duvel bottles should be good to at least 3 volumes CO2, probably higher. One thing I do, if I know I'll be drinking some of the beer young, is to only use heavy bottles for the ones I'll be aging (and to plan aging based around how heavy the bottle is, e.g. a champagne bottle should be good for a long time).