I'll bite too.
I'd worry less about getting a good stoner doom amp and more about getting a good amp.
Any good bassist will be able to get a decent sound for their style from just about any standard amp.
So what's more important to you I guess your budget will guide most.
First up, head & cabs or a combo, how will you be transporting it?
Combo's are great for portability but Head & Cabs are great. If you want a thick low end sound then larger speakers in the cab(s) would help but saying that ampeg's 8x10 cab has awesome low end.
Personally I'd recommend going for a Head & smaller cab (2 or 4, 10" or 12"speakers) now and looking at adding a larger cab (maybe 2x15")later.
How loud do you want it, I'm guessing loud. if your guitarists are rocking 100W amps then I'd recommend an amp over 200W.
Go visit a music store or website and see what fits those criteria within your budget, try them out if you can or look for youtube reviews. Don't forget 2nd hand, your money will go further.
As regards a pedal you'll likely want a fuzz pedal.
However a graphic EQ pedal will help you tweak any amps sounds.
My personal recommendation would be to get a cheap decent multiFX Bass pedal with a tuner, Fuzz (graphic eq if possible but that's unlikely or cheap) and with a DI out. That will cover so many eventualities when you start out and you can then look at replacing it with better versions of the specific fx you actually use.