sandwhale
welcome to the forum....
I take it form your handle sandwhale you live or you would be useing the buggy on the beach?
Wheels... on compack sand you can get by with 4x8 wheel barrow type of set up in the USA avg price for the tire is 16.00... 3.00 for the tubes and
for the nylon wheel 15.00? maybe... OR for deeper sand the big foot tires 8x21 are a good choice more money but can fit the same nylon hubs
Stainless is the way to go for the salty beaches, steal is another choice but the metal is going to need a good power coat or a butt loads of sealer
and primer then paint to keep from rusting.
seat can be made from 2"webbing like a seat belt...
Best to go and buy.. time spent chasing down material, welding making a seat, wheels bearings and spacers could be spent buggying... also yor going
to have to figure out were to weld the foot pegs, rake on the front fork and bends for side rails, padding..... un-less he's in prison and has
plenty of time to kill and then if all work out maybe it will ride good.....
for a couple of hundred bucks { US funds} could get your brother up and running