The main things you need (besides a decent tranny cooler) is to make sure line pressure is decent, do the sprag upgrade, and add the extra center support bearing. These make the biggest difference in durability of the tranny.
A manual valve body is a huge asset for off-road use because it allows you to lock the thing into 2nd & 3rd gear low range for various obstacles. Just make sure you retain engine braking or you're most likely gonna miss it. Many of the full-manual valve bodies, especially the reverse pattern ones, eliminate engine braking in at least some gears. The upside of the reverse pattern vavle body is that you can quickly toss it into reverse from first to pull out of a potential roll.
Other things I run in my TH400 are additional clutches in the forward drum, Kevlar clutches & kolene steels, A reworked front pump, hardened input & output shafts, a deep pan, and a 2500 stall converter. These aren't necessary but do help keep the tranny alive in really abusive environments, such as competitive rock crawling.
Another important thing people overlook is the flexplate and the converter bolts. These are not the place to pinch pennies, because for an extra $75 you can get the peace of mind a SFI flexplate and quality converter bolts. Far better than mangling your flexplate or breaking converter bolts on the trail.
You can also swap in the TH475 straight cut gearset to eliminate the gears walking inder load. To me, this is not necessary except for very heavy vehicles with extreme power input.
If it was me building it for recreational and light comp use I'd do a good cooler, deep pan, a full manual valve body with engine breaking, more clutches in the forward drum, a 2000-2500 stall converter, SFI flexplate & good bolts, & the sprag & center bearing upgrades. This should net a tranny good to at least 500hp on a reasonable budget.... and is already overkill for 90% of 4wd enthusiasts. If you manage to smoke that, you're into the realm of big bucks.
Hopefully I'll bump into you competing sometime... growth is always good for the sport.