Buy US van from the 90s/beginning of 2000 – open questions

Hello together, The topic is not new, but the info in the threads I found are either too old or were treated only marginally. I would like to buy a US van from the 90s/beginning 2000s. Here come GMC Savana/Chevrolet Express (from 1996), Dodge Ram Van (from 1994 or even 1998) or Ford Econoline (from 1994) into question. I was first very focused on the Dodge. The frame construction (uniframe) and especially the rust problem have me alldi The Ford Econoline seems to be the “robust” of many of the three vans, but is rarely found in good condition and the spare parts supply does not seem to be as good as with the others. Stays GMC Savana/Chevrolet Express (unless I have a fancy Ford running over the way). Also with GMC Savana/Chevrolet Express there is the rust theme, but obviously not as bad as with the Dodge – but maybe this is only due to the al Lg. better condition of the offered models. What about the “Canada model”, which is galvanized and therefore rust-resistant? Does this also apply to the German versions with kmh-tacho or even the imports from Japan (also kmh Tacho)? Then I stumbled again and again over the fact that it is recommended to retrofit a cooling for the transmissions when driving a lot of highways. What is it here? How fast should you drive maximum without such retrofitting?