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 www.cadplans.com 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