You can also cut an axle one down if you find one used for cheaper too, but stub axles work great. When i built mine i just used some wheels from harbor freight, never flats designed for a wheelbarrow, and just used the appropriate sized bolt for the axle. You could also buy replacement wheels for an arbor trolley, which are about perfect for loads up to 1k. But trailer hubs would be best, i was keeping it so it will pass thru a gate and be human powered. The bigger the diameter the easier the thing will move.
I'm sure you are thinking you want to build it so it flips up and everything, which is completely overthinking it. If you get some log tongs that have a grab hook, you can simply weld some chains on at different places and then you can move them if you need more leverage or a higher lift. Then you can get 2 grab hooks and put them pin to pin, and then you can use a slip hook logging chain and move other non log things, i use mine all the time to pick stuff up that's too heavy to do by hand and move it around. I'm talking about the "fetching" style, which on mine is used on loads up to about 1k with 2 people on the handle, if you are doing a vehicle mounted one you can use a winch, but that would be way slower for the smaller sized logs (under 2').
If you are building a bigger one make sure you don't skimp on the winch, and get one that free wheels (in other words not a worm gear one). I would even contemplate getting a 2 speed one, and setting it up so you could do short winch pulls to untangle stuff so you can hook up to it since an arch has to straddle the log to work. Maybe even a large sailboat capstan winch so you could use rope on it, possibly in addition to a wire log lifting one. Look at the old school dozer skidding arches to see what I'm talking about, a truck pulling one set up like that could pull some huge logs. I know you are wanting one arch to be able to do both easy hand moving stuff and vehicle towing stuff, but honestly there's a reason they don't sell one like that because the more capacity you make it have for towing the harder it is to move by hand. You really need 2 of them, a hand one designed to be used by hand with a fetching action, and a huge towable one designed to be used with a vehicle. If i was trying to get logs out for milling, i would be leaning towards the towable type, but since i needed one to get back yard trees to the truck i built a small fetching one. Make sense? Basically no matter what it will have to compromise hand moving ability to be able to move bigger logs with a vehicle. If you are building them, they are cheap enough to build that you would be money ahead to just build two rather than buying one.