They will benefit from a tree tube while they are young if you are in cali planting in the full sun. Pawpaws are natural understory trees. In a forest succession, they would be the 2 or 3rd in order. They would grow in the shade of larger trees, when those trees die off the pawpaws getting to a mature age would then be exposed to full sun.
No not really if you trying to plant in a pot long term, they have really long tap roots like a carrot.
Look at this post - Why it is hard to transplant pawpaws?
If you can afford it, that’s probably the most optimal choice. All the online nurseries, OGW, Peaceful, Raintree, starks, sell small potted grafted pawpaws (they are small precisely because they don’t like being in pots too long, much like nuts). The only one that sold bare root was Cliff (@KYnuttrees ), but he only does pick up now. You might be able to find some larger like 5 gallon size pawpaws at a large nursery, I know they occasionally stock them. I believe that is how @Vincent_8B found them in the seattle area.
stark uses 4x10 band pots and larger, which are bigger potted pawpaws than you would usually get from OGW and RT who uses smaller 3’ or 3.5’ band pots. They also usually come larger than I have seen them come from Peaceful. (Perfect circle farm - another decent source - also uses 4x10 band pots if I remember correctly) That said, it would still be considered unusual to get fruit from a starks transplant within 3 years. Pawpaws don’t really runt out, but if you let them fruit early, they will stall in their growth and take longer to get to appropriate size.
A pawpaw in a 4x10 pot would usually be 10" tall at least: a 2-3 year grow out - 1 year from seed, 1 - year grafting, maybe another year to grow out from grafting. From seed pawpaws usually take 6-8 years, so a potted/grafted may get lucky and shave off about 3 years and bear within 3 years from planting. Though usually it would be 4-5 years because they are so fickle about being transplanted they need a year to settle in the new spot. There are no dwarfing rootstock for pawpaws, so really no way to “cheat” and get early fruit. Everything is full size and slow to bear, one of the downsides if you end up not liking the fruit. Once you put at least 4 years into growing it, any sane person putting that much time into growing a tree will feel aggrieved to cut them down ( Removing fruit trees: what and why? - #298 by Bigdoug03)) even if they want to make room for something else.
As far as cultivars, you ask 10 different people and you’ll get 10 different answers. It’s like asking about favorite apples. Some like tart, some like not tart, some like sweet, some like not sweet, etc… Cliff (@KYnuttrees England Nursery owner), Buzz (Perfect Circle nursery owner), @marc5 (Valley View pawpaw orchard owner) all seem to prefer mild pawpaws. Same as me and @TrilobaTracker . @Vid @ramv enjoy heavier flavored pawpaws it seems. @disc4tw @TrilobaTracker seem to gravitate toward mild to medium. @IL847 @SMC_zone6 seems to enjoy them all. You’re not going to get a single straight answer on this. The other thing is that since pawpaws are so slow to bear fruit, most people that try pawpaws have tried only either heavy or light, so they are making an early judgement call on whether they like the fruit. They may hate heavy but have only tried light-flavored cultivars and just assume they like them all and plant a bunch of heavy flavored ones. Conversely, they may have tasted a heavy-flavored cultivar and decided they hate them all, when they would have enjoyed light-flavored pawpaws. I find pawpaws to generally be probably the most polarizing fruit that can be grown in temperate climates. You either LOVE them or you HATE them. There are some people that are indifferent, but they are much fewer in comparison (like as a percentage) compared to apples, pears, citrus, etc…
Another consequence of it being slow to bear is that cultivars take time to get shared around. It’s just within the last 5 years that Shenandoah is like the most common pawpaw you can find growing in a backyard orchard, but it was patented more than 20 years ago in 2001. All popular cultivars are at least 15+ years old. The latest and greatest stuff which do taste much better imo, rarely get mentioned simply because they are new. This doesn’t happen with say apples.