where/how do I train with Crane
If you go to the desert zone where Yamcha and Puar live, in the extreme northwest corner of the map there's a small pass through the forst that leads to his house. If you haven't trained with Rosshi he'll offer you a fairly difficult level 27 quest. Complete that, return to him, and he'll train you.
Important: after you kill the wolf, heal up before leaving the cave.
and Karin?
From Mr. Satan's Estate, go directly east through the pass between the mountains and trees. Follow the path to the next zone south and you'll come to Karin's tower. Bosa and Oopa's dialogue doesn't appear to be implemented in this version, but all you need to do is climb the tower to get to Karin.
Note that climbing the tower is non-trivial if you don't yet have a means of flying.
I keep getting characters to the point where they max
out on two schools, but die before level 20
Character build is a significant part of gameplay. You can't just make a character and level him. Some things work better than others.
I should probably just train at three schools, but some builds will max out prior to
level 20 I imagine (some barehanded builds in particular seem to max out quickly).
The way I remember it, three school builds were generally more common, but it depends on what your goals are. The fewer schools you train at, the more schools you can destroy to increase your late-game skillcaps once you start getting dojo destroyer quests. But yeah...obviously you want to survive to get to that point too. Early game character design is generally a matter of prioritizing among armor, hit points, speed and damage. There's no easy way to get all of those without taking three schools, and even with three it's often a matter of choosing only three out of those four things to focus on for lack of skillpoints to invest in all four.
some barehanded builds in particular seem to max out quickly
On barehand skill sure, but I think not having enough skillpoints is generally more of a problem than maxxing out and not being able to spend more. Remember that constitution increases your hit points, dexterity increases both your to hit and armor, and martial arts skill increases your to hit and makes life easier by increasing your ability to identify items.
In any case, some schools synergize together better than others. For example, if you want to go barehanded, karate gives blocking which increases your AC by an mount equal to your barehanded skill per each available hand, but kickboxing gives double attack which allows you to use each empty hand as a separate attack. And Judo gives access to the dodge skill which gives a raw AC bonus.
If you're going for a high armor build, it makes sense to take karate with judo because they'll both enhance your AC. And, since neither school gives a constitution bonus, you can take ballet for constitution and pick up the jumping skill in the process. Karate+Judo+Ballet is a strong combination if you want an extremely durable character. Quick check...at level 16, I'm able to make a character with with 71 AC while naked, +14 speed and 224 hp with karate+judo. Add generic, unenhanced trenchcoat and heavyweight from the shops I can get AC up to 111. +26 to hit with 1d20-4 base damage. Completing the ballet quest with those stats is difficult, but add a couple levels and better gear, and it should be easy, opening up ballet for constitution. Back in the day, I remember players using this build to get armor in the 200-300 range pretty easily. Note also that while this is a fairly slow to get powerful build...I'm trying it right now, and at +14 speed I can walk in circles around Honda's sumo ring without him ever touching me.
Alternately, you could study kickboxing for barehand skill and double attack, plus sumo for strength and constitution. Checking potentials right now...at level 16 I'm seeing a character with 20 strength, 34 constiutution, two empty-handed attacks for 1d20+10 damage each, with 544 hitpoints. Sumo also gives access to Tameshiwari and Immovability, both of which are cheap and useful abilities. Add in the slightly but acceptably skillpoint-inefficient speed training from kickboxing, and this is a viable two-school build. And by choosing high health over armor, you can take full advantage of the running bonus to speed without any real loss since you're not really trying to avoid hits anyway. It does suffer from low to-hit values, though and could possibly be improved by adding in kung fu for the martial arts and dexterity training.
And in all cases, you'll want to consider which abilities are or aren't available later from chi trainers. For example, like you note, Rosshi teaches Double Attack. So if you go karate+judo+Rosshi, you can learn both block and double attack. Whereas the crane hermit teaches blocking, so if you go kickboxing+sumo, you'll probably want to train with the crane hermit instead of Rosshi. And any build that takes sumo requires that your character be male, which excludes the possibility of marrying Oolong.
There are many viable options, and a lot of exclusions to consider. But, for simple barehanded builds, these two were the most popular back in the day.