Welcome jifop.
Those Peter Lynn wheels should be perfect with 20mm axles.
My chassis ended up being made from 1.6mm wall material and I found 2mm welding rods were the best for welding with a fairly low amperage using a little portable inverter welder.
If you keep your welding arc fairly short with the 2mm rods you don't end up blowing too many holes in the material.
If you don't have a welder yet I highly recommend spending a little extra by buying a MIG welder. Better still if you can borrow one.

Lincoln Electric is one of the most highly respected names in the welding game. Their Power MIG 180C is an excellent choice capable of gas or gasless welding plus a spool gun for aluminium welding can also be bought. They retail in Australia for $1,200 so should be around 600 Pounds in the UK. Try www.lincolnelectric.com.au to have a look at it.
What ever you buy it is a good idea to buy from a dedicated welding supplies shop rather than an auto accessories shop. A welding shop should have a demo bay where you can try out the welders.
I didn't plan for my yacht to built from 1.6mm material. I thought I had 2mm.
So far it has proven to be quite strong enough and the yacht was built almost exactly to the plan. My personal variations to the building plan are being corrected except for the material weight.

As Paul said when he saw it and had a sail "The 1.6mm material experiment has been done and it works."
If you build it exactly to the plan you will have a great yacht. Happy Building. Cheers Cisco

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