Hi Mikey
I have two of the SK800 both in a set , a german one with red german cars and one with the international version (blue cars, roof with ribs)

Your questions

1 yes the spoon current pickups need replacing with a normal slider. Very easy

2 Both mine have no tires (approx 1951) and the pulling power is OK. Remember in those days, axles were "Blunt" (no thin needle to ease the friction) so it was designed to pull 4 or 5 cars without any problem

3 Zinkpest was not a problem in those years. It was rather during the 1946-1950 period were Märklin used recycled / contaminated metals provoking intercristalline corrosion

Here is one of the 2 SK846J (International) startersets I have


For sure your loco is not 1959 as the last year was referenced 3006 and had traction tires the year was 1957. Traction tires since 1955
Spoons pickup until 1952 I think

Cheers
Jean