A few tips from a fellow backhoe enthusiast here. Just what I can think of spontaneously:

- Try to design your backhoe to have smooth curves without sharp corners. Taper any reinforcement plates to make a diamond shape and use a tail-out on your welds. This helps to reduce stress concentrations and fatigue cracking, which is a big problem on backhoes. Like this:
http://www.millerwelds.com/resources...etch?id=507931

- You will not regret having power slew, even on a really small machine. There is so much inertia in an extended backhoe that it takes a lot of torque to get the backhoe moving. I just checked the manual for my 7-ton backhoe loader and it gives the slew mechanism torque: 4860 ft/lb.

- Think about how comfortable the machine is going to be. Get a two-lever valve block if you can (these are available second hand). Make sure the sitting position is comfortable. And make sure you can see down into the trench clearly, because that's a problem on a fair number of machines.

I've got a lot of backhoe information if you need it. Just let me know. And do show us the finished machine!

Chris