Slow and heavy Iommic riffage
I currently play mostly C standard with "Not Even Slinky" 12-56 strings. When I switched from E I was surprised at how easily the guitar tuned, played and intonated on C with these strings. Didn't even need to adjust the truss rod one bit.
As far as guitars I think it doesn't really matter as long as you like the feel. You can always change pickups, tunings, strings, FX, amps.
For guitar "feel" matters so much more than anything else so go with your heart/fingers.
I like humbuckers and they are the more standard choice for big fat doom tones, but single coil pickups are cool too especially with fuzz. I like Stringjoy strings personally so that I can get weird and specific with gauge/tension, but if you are going for D-C tuning somewhere between 52-56 is the range I would use. For reference I run 60's for my drop A non Gibson scale length guitar. As for what tuning to use I'd offer that with C# standard you can play so many great Sabbath riffs, and with the turn of a machine head you've got access to drop B.
Definitely get a Behringer SF300 pedal. It is a cheap copy of the Boss FZ-2 Hyper Fuzz and will get you that Electric Wizard tone. The ProCo Rat itself is pretty cheap if you have those available. Other suggestions are anything in the big muff family, for a cheaper option there the TC Electronics Honey Pot is basically a green Russian muff to my ears.
I always like best-of-both worlds options, so I'm going for a guitar with good high output humbuckers and coil splits on a push pull pot. Ah yeah I did like the sound of C#standard when I tried it on an acoustic guitar this week, and I hadn't thought about being near Drop B. That's a bonus! I know about the Behringer Super Fuzz, but I really don't like the way it sounds. Feels like rattling a tin can of nails. Or angry wasps. I do like Fuzz Face style fuzz, and Muff style, but I want something in between them. That's where the Carcosa goes. I listened to gear demos and reviews of a million fuzz pedals on YouTube and I cannot think of any other that I liked more than that one. The actual ProCo Rat is a bit hard to get here, that's why I was looking into the Joyo clone. Other than it having the issue of not working well under intense heat, it seems to sound just as well as a regular Fat Rat. And I don't intend on playing under the summer sun at midday anytime soon.
Just boot up DOOM if you want to play it. The tracks are pretty metal.