Smaller isn't better when it comes to finding gold. You're thinking to small for Idaho, you want portable then go with a 4" submersible with a Keene P100 pump and just go for broke. A 2½ has merit as a sniping dredge but that is the smallest. A 1½" dredge isn't something that I would even start my 5 year old grand daughter with.
You have never dredged and we have told you what's up. My partner Jim was designing a 4" dredge to run off of twin P100's for the weight. You're in Idaho where you have some really good gold areas. We know what you are thinking Brian and because we have dredged for decades with all different sizes of dredges from 1½ to 10". I see your in WA so you need to first verify that where you want to work is open to dredging. I did of course build my current dredge (6") and I've saved $1000's over a used one but when you start increasing in size the price also seems to double to triple even on used ones. Sorry if this is not what you want to hear but I've been there, done that, learned from my mistakes, spent more money trying to cobble something together then I would of just buying a used one. Also if your married, then you'll have the wife saying "Your spending more money" even though in your mind you justify the expense.oh wait maybe that my wife lol, but you get my point. Plus the lost gold, fuel spent and frustration. By the time you buy all the needed parts you'll have spent between 5 bucks PLUS your time to build and test, tweak it, further test and so on. I can tell you that you'll end up being much further ahead if you buy a used dredge off craigslist.