Depends what club you are competing with.
If in doubt, buy a weld together kit from QT or North Off Road.
No it doesn't. The minimum MSA spec is BS1387 (known as 'blue band') 48mm outside diameter with a 3.2mm wall thickness. BS1387 is usually sold with a nominal bore of 40mm.
That assumes he is talking about an MSA Affliated Club, although I would go with one that isn't.
I have come across many people who feel there is no point in joining a club until they are ready to compete, and build the vehicle from information they have got second or third hand, then join and find they've been given wrong info or not told of a rule change