Hi, welcome to the forum.
The first thing I will say, is that there is a chap called Robert who has a YouTube channel - he has pretty much undertaken and fully described every repair you could imagine.
https://www.youtube.com/c/RobertDIY/search?query=850
I've used the 850 model in the search as your V70 will be the 'pre-ME7' version of the car - it will have a cable operated throttle body, not the later electronic one. Your car is pretty much a cosmetic makeover of the 850, so most of the 850 info will apply to your car.
Regarding brands for the PCV... the oil separator box doesn't need to be OEM - PartsForVolvo do a copy which is fine. You could even just check the one you have, maybe clean it with degreaser or carb cleaner etc and re-use. I'd say the only OEM parts you shouldn't scrimp on are -
* The hose from the separator box to the cylinderhead - it connects near the oil filler;
* The small hose that connects the separator to the engine block;
I've found aftermarket equivalent hoses either don't fit as well, or split within a year or two.
If you're keeping it (PCV) standard, then I can say that I've only used the OEM long hoses - they lasted 20 years, so they must be ok right? Expensive yes, but they can be considered almost fit and forget if your engine is in good condition.
You could consider doing a PCV upgrade using a 3/8 hose as described here -
https://www.matthewsvolvosite.com/fo...ic.php?t=53448 Recommended for high mileage cars, or on engines that are worn and have a bit of cylinder blowby. I did it on an S70 T5, but tbh getting the larger hose to sit comfortably under the intake manifold without kinking was a real test of patience - nowadays I'd use come copper pipe instead
Re the fuel filler flap issue, take a look at this, it describes the components nicely
https://www.youtube.com/watch?v=ZvUht8nz8jw It applies to both saloons and estates - you will also have this lovely wire pull release cable, its hidden behind the rear lamp/bulb access panel
Hope this helps
BTW, I'd also suggest an autobox fluid replacement - its rarely done by garages but should probably be considered every 60k miles or so to protect the transmission. (This will generate a debate as there are reports of this 'killing' transmissions - its not true, but fresh fluid can highlight a badly worn clutchpack in an autobox). Fluid will be Dexron III in your car.