hi Lonesome-Whizkid,
we seem a little confused
hmy: that is not suprising with the amount of kit available,
Andrew was refering to the caravan chassis in most cases the max hitch limit is 100kg of noseweight this simply means that say you attach the caravan to a lorry that has a max noseweight of 500kg the actual maximum noseweight you could use would be 100kg as this is the limit set by Alko. if however you attached the van to say a Meriva the max noseweight you could use would be 50kg because this is the maximum for the cars tow bar, you always use the lowest figure what ever that is.
next there is no difference in the flange fixings or size, on any tow bar ONLY the ball is different with a standard flanged ball the diameter is 50mm however it has a short length neck, this will foul on the alko hitch of the van if one is fitted,
the Alko ball is also 50mm and has the same bolt holes BUT the neck is longer so it does not foul the hitch.
if you decide to fit a detachable tow ball (swan neck) these are fine with the Alko hitch because as the name implies it has a swan neck and will not foul the hitch either.
next if you buy a car that has a bar already fitted you could garuntee it would not be rotten they are just too strong the only rust problems could be in the tow bar mounts on the body of the car seeing has you would probably want to tow with a car under 10years old I doubt there would be a problem there either but you could have it checked on purchase on the previso that you are going to tow a caravan with it!!.
next all towballs are removable however if you have the alko flange type fitted there is no need to ever remove it as these are also suitable for a van that does not have a alko hitch but the old greased type, this is what I do as I have two trailers one has the alko (van) the other a box trailer with greased ball both work just the same on the alko ball BUT after using the box trailer the ball has to be completely cleaned to remove any grease as this would ruin the friction pads. if you get a detachable tow ball (swan neck) these need no spanners or nuts as they are quick release and just click into place, this type is also suitable for either alko or greased hitches.
next I am not supprised they look the same to you as they ARE the same only the length of the neck on the alko ball is longer. (as an aside) the ball covers are unique to the different type of ball a Alko cover only fits a alko ball and visa versa, (because of the length of the neck) SEE
hope this helps and answers some of your questions keep them coming as this is the best way to learn