You said for a small diesel, so we are talking about mechanic stuff. I'd start with the basics. Socket set with extensions, ratchet, and breaker bar. Unless it's a really small engine, like a small sail boat engine, I would get both 3/8" and 1/2" sets. I think all engines now are metric, and have been for quite a while, even the US brands. I don't know exactly when they switched over, but Cat, Cummins, Deere are all metric now. And certainly Yanmar, Volvo, MTU, MAN, Scania, etc.
The screw drivers, side cutters, pliers, big channel lock pliers, a hammer,.
Then a set of combination wrenches.
And a good Volt-Ohm meter, and a laser thermometer. A clamp on DC meter is very handy too, but be certain it does both AC and DC.
That will get you pretty far, and from there I'd just buy what I need, as I need it. I've been wrenching on stuff for over 50 years now, still have some of the first tools I bought as a kid, and am still buying stuff as I need it. Torx heads (both male and female) are increasingly popular, for example, as are hex socket heads. If you encounter them, it's time to get a set.
You can also improvise a LOT with basic tools. For example, I don't use oil filter wrenches, and instead just use channel lock pliers.