That's true, it can't be repaired. The reason it leaks after you shut it down is that temperatures rise (coolant is no longer flowing, air is no longer moving through the radiator) and then pressures rise.

This is a link to a DIY at BW that looks pretty complete. Radiator Do It Yourself: You'll Get Your Hands Dirty! - Benzworld.org - Mercedes-Benz Discussion Forum

This is not an awful DIY. Oh, and unless you're going to change your ATF, you won't need more than a quart/liter of fluid, but you will need a way to check the fluid level afterward.

Also, if you plan on keeping the truck, I would suggest you get one of the good name brands to replace your radiator, at least that is a known quantity...and quality.

Good luck.