It is important to think carefully about what you should use hammers for and also try to look ahead. Typical is that after a few years use the machine you first bought or built is insufficient or is inefficient in some way. You mention a 40 kg hammer and I agree it is a good alround size. It can forge steel up to 2 inches great. Its ofcourse possible to get bigger pieces between the dies, but the effect is not so big and it takes longer time . I have considered building a hammer, but since I started early with air hammer, I was afraid that the result would not be usefull enough. It also cost much with all steel and parts needed for a build here in Norway. So the first hammer I bought was Anyang 15 kg. It was much smaller then the 100 kg Becher that I have used at work. The 15 kg is small and handy and beat relatively hard. About 300 hit per minute keeps the iron hot for some time and much can be doen in one heat. Forging 25 mm round iron is no problem. 30mm square gets a little tough for the 15 kg Anyang. I sold the 15 kg after one year or so and got a 50 kg Demoor and later also a 40 kg Anyang. I have had some trip hammers. The works okay, but gives me a sense of a museum piece.. Not to be negative, but air hammers are more my thing. I have had several types of air hammers (dont know the exact number), old and new. Old is extremely heavy built and usually two piece. You can get new self-contained air hammers both one and two piece and I would probably gone for a two piece hammer. The to piece hammer can be mounted on a steel base like the one piece , so instalation is the same. Its great to have a seperated anvil that is heavy. Ram / anvil ratio 1/10 .