Weight of everything at the truck scales or recycling center is a great idea (Truck scales might be easier if you can borrow a large trailer, weigh the trailer empty and then set the parts on the trailer and reweigh.
What type of brakes are on the axles the guy has for $100? If they are electric then all you need is a electric brake controller if you truck does not already have one installed. Electric brakes are probably from a travel trailer. Boat trailers will have hydraulic brakes since they get submerged all the time. For you purpose either will work. If they are hydraulic, it will either be surge brakes or electric over hydraulic (surge is most common). But you have to have the surge coupler to go with the hydraulic surge brakes to make them work. Electric over hydraulic uses a brake controller just like the electric only do and it has a component on the trailer that converts the output into hydraulic pressure. I think standard electric would be the easy way to go for your trailer as you are only dealing with weight and not water like when it was a boat trailer.
Also look at the thickness and size of the trailer frame to make sure it can handle the weight also. That smoker will weight a bunch more than a boat that lived on a single axle trailer ever will.
One more comment on trailers. If your truck has a 7 pin RV type outlet, then I would wire the smoker trailer with a 7 pin plug to fit. That way you don't have to fiddle with a 7 pin to 4 or 5 pin adapter. The less connections in the chain the better, plus it's easy to loose the adapter, have it wiggle loose, etc.... I've converted all my boat and utility trailers to 7 pin to match the hitch on my truck as it was installed when I had a travel trailer. Just makes it simpler.
