Not all Q's and A's, but for clarity's sake:

Q: I might be getting a '65 P1800.
A: Congrats! Check it over carefully for rust. The top three issues and problems with 1800's are rust, rust, and rust. Well, maybe replace that last rust with crash repair, body shops almost universally seem to screw up old 1800's when they try to work on them. It's just a lot harder and more exacting work than they are used to doing on newer cars. Pretty much everything else (interior, mechanical) can be redone to as new condition with some money and time, not necessarily so the body.

Q: It has a B18 presently with O/D tranny.
A: Nothing wrong with the B18 at all, unless it is worn out I wouldn't bother replacing it with a B20. Stock for stock, you won't notice much difference. Of course, if you are planning on warming up the motor somewhat, a B20 is the logical starting point.

Q: I have a B20 I want to stick in it. Will it bolt on?
A: Yes. 100% bolt-in. You may need to block off some injector holes in the head if it is from a later FI car. And when you bolt the B20 up to your B18 flywheel, you will notices that the bolt holes on the B20's lower bolt on aluminum brace dont' match up to anything on the B18 bellhousing, but that's OK. I've run a pretty warmed up B20 like that for years now with no issues.

Q: ALSO - were the '65's 6 Volt or 12 Volt?
A: 12 Volts.

Q: Would rear disc brakes off of a '73 144 be adaptable to replace rear drum brakes on the '65 1800?
A: the word 'adaptable' has many connotations, but in this case I'd have to say 'not easily at all'. The axles just mechanically changed a lot. Your best bet (if you really wanted rear discs for some reason, although the drums are entirely adequate) would be to locate a late model 1800E/1800ES rear axle. That should come close to bolting directly into your 1800S as a complete unit (I'm not sure if 1800's changed rear suspension designs in '67-ish time frame like the 122 sedans did or not). Then some fun with driveshaft flange sizes and Volvo's penchant for changing them on a nearly random basis, some fun with hand-brake cables. And last but not least, fun with the changed bolt pattern (older rear-drum braked Volvos have the very common 5X4.5 inch pattern, newer rear disc'ed Volvos have 5X108mm (roughly 4.25 inches) patterns which they share with all newer Volvos (to this day), certain moderately newer Ford products (and their platform sharing Jaguar twins), Ferraris.
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I'm JohnMc, and I approved this message.