For speaker cabs I recommend the highest wattage you can get away with. DV Mark's Neoclassical cabs (1x12, 2x12 - what I have - and 4x12) have 150-watt Neodymium speakers that stay tight and clear with whatever volume or pitch I throw at them. A little speaker breakup at higher tunings is nice, but the lower you go, the more mushy and unpredictable it becomes. The DV Mark cabs are a little pricey new, but can be had for pretty good money used (I see the 1x12 pretty regularly in the $300 range).
As for amps, you ideally want something higher wattage not because of the volume but because of the headroom. With lower tunings, you want as clean power as you can manage, or you'll lose both clarity and projection with your low notes. A 5150 III 50-watt (I like the EL34 one, but either way make sure you get one of the new ones with separate gain and volume for the blue and green channels) will cover a lot of ground, but the lowest volume it reasonably does is still "pretty darn loud".
If you want to make a bit more of an investment, the Hughes & Kettner Black Spirit 200 sounds really really good, has a ton of functionality that works great with lower tunings, and gives you 200 watts clean of class D power. It's about as loud at the end of the day as the 50-watt 5150, but the volume range is a lot more usable. I have the Grandmeister Deluxe 40 (basically the 40-watt tube predecessor) and the voicing (which the Black Spirit keeps, it's a pretty signature H&K thing) is killer for lower stuff. The DI on both of these is basically the best built-in solution you can get, so if you want to do any recording it's a great option and scales up very well.
Any of these will be like twice as loud as your Mustang though, but the H&Ks will both go pretty quiet. They all also have cleans ranging from pretty-darn-good (5150) to really good (Grandmeister) and will cover the full breadth of baritone sounds I can think of off the top of my head.
