i’ve always gone for the lychee with the thickest trunk, as long as it looks like it is in good health (as demonstrated by recent growths that look healthy), even if the canopy isn’t huge.
i’ve found lychee to be able to give vigorous growth after pruning or damage (eg heat/drought damage) - ive harmed many lychees by accident and they die back a little but put out vigorous growth. the thicker the trunk, the more vigorous the growth. the main danger is if it happens in the fal, the tree will go into dormancy instead of push regrowth. another danger is if the tree isn’t protected from wind or has salt burn (use osmocote plus or foliage pro, use rain water RO or filter) it won’t grow back well.
if the canopy shape doesn’t look ideal, just let it grow wild for 1 season to let it get established, then in the second year prune the canopy as soon as you see the first vegetative growth of the spring (if winter is over).
my lychees don’t seem to mind wet feet, and almost all were mildly root bound at purchase but gave healthy growth with wind protection, rain water, good soil (pH 6), and foliage pro fertiligation (with half dose osmocote plus)