Hardest part is the large number of choices out there. If this is your first foray you might want to consider a tank with drop in coils before tackling an RTA. Not that it's all that hard to build, but you're looking at learning a lot of stuff real fast.
For a mod, a dual battery can serve you well. Make sure you get quality, name brand batteries from LG, Sony, or Samsung for it. Two 20A batteries should be good for 120W of use, which is more power than you'll likely ever need. Dual mods are frequently rated up to 200W, but that's really best achieved with 30A batteries and honestly not at all necessary for even the cloudiest of builds.
Actual recommendations are best made based upon your budget and location. No point worrying about brands if you can't get them. I would generally recommend something from Smoant for a quality midrange mod. The Cylon is an excellent example of an affordable mod that has all the bells and whistles and a decent reputation for quality of build. I'm not a huge Wismec fan because of varying QC, but do have both a 3 battery 200S and a Pressa 100 that work just fine.
Tanks, well, lots of personal preference there. I don't have a Manta but it has a decent reputation. If you're new to building I'd be looking for an easy deck to build on and maybe something with top air to reduce the aggravation of leaking which is common starting out. RTAs like the Zeus, Juggerknot, or an OBS Engine come to mind. I would definitely read through the individual threads for different tanks in the RTA section on this forum. Same for mods. But at least you have some general ideas of what you look for.