I actually built a machine very similar to this one a few years back and had a terrific time. It ended up being one of the most fun things I've ever done, and I still use it quite often for digging around our yard. Some points to consider:
1) Generally the hydraulic pumps and engines are matched to the design. This design looks quite similar to my
628 backhoe, and the 18 horsepower Wisconsin may be a bit much for it though I don't know. My machine is powered by a five HP Tecumseh and looks almost exactly like this one. I'm pretty sure that if I put 18 HP on my machine, I'd bend the boom like a pretzle.
2) Check out the cadplans site. I think Jim Mikulous (sp? I think that 's his name) has been designing these machines for a very long time, and I found his directions superior.
3) My sense after building the smaller 628 was that if you get much larger, you really should look at rebuilding a used machine, but that at the smaller end it actually does make some economic sense.
4) Use high strength welding rod. I used 7018 electrodes. I would not mig weld this together with a small light duty box from Walmart/sams club/kmart/whatever.
5) I would build a narrower bucket. I still might for my machine, which has one around a foot wide. For this tough Virginia clay it does o.k., but I know that the 5 inch bucket would really be the cat's meow for trenching and planting stuff.
6) It really does work. I dug the foundation for our house addition with it and saved a bundle of money, so it actually did pay for itself when all was said and done. Makes a hell of a conversation piece too.
Good luck, Charles Morrill