It's plenty strong, and one of the most popular manual light truck trannies ever built, if not THE most popular. Only a six speed NV5600 or G56 (supposedly) is stronger unless you get to into medium and heavy truck transmissions.
As mentioned, it does have some weak spots that can be addressed. When the local transmission shop fixed my 5th gear nut a few years ago, they warned me they were not sure the repair would last long. "The shaft is pretty worn from the 5th gear rocking loose on it for so long; not much spline left. " It's still going fine, but a fully-splined replacement shaft and nut kit would definitely be in order the next time.
I don't tow heavy over mountains, but I do plow snow. I seldom use 5th gear for that... :-laf
Before the 5th gear nut/mainshaft gives out again, I'll already have the NV5600 and NP205 in, I hope.
For what you are doing, finding a complete NV4500 conversion setup all from one place is real bonus and you can always later upgrade to a 6-speed if you find the NV4500 is not up to the task. And that package is more complete than any I have ever seen, right down to the steering column to get rid of the auto shifter.
After reading that auction more carefully, I see it is for a 24 valve 2nd generation. I can't think of a reason why it would not work on your 12 valve, but maybe I'm missing/forgetting something that someone here can point out?