Diesels are harder. It’s not an issue of the load on the engine, or any additional complexity with the gasifier. The only difference is controlling the engine. You need a source of ignition, either a small stream of diesel as a pilot, or convert it to spark ignition. These are both non-trivial for the beginner.
Also keep in mind the compression ratio. Some newer engines run very high, around 22:1. Above 17:1 the woodgas will detonate, which will throw off the diesel’s timing and make it run very rough, even damaging the engine. The compression can be lowered using thicker headgaskets. Again non-trivial for the beginner. But it can be done.
Veggie oil will not make any difference to the woodgas, however it has its own issues for cold starting. I’d stick with plain diesel for the pilot stream unless you know exactly what you’re doing.