Help

1989 Vanagon GL Westfalia

last night i had a big blowout on my left rear tire on the highway..pulled
over and after the smoke cleared and i dropped my spare i realized it is all
dried up, probably under the van since 1989.

so i need at least 2 tires to replace, i am running on 185/75/14 goodyear
somethings,

1) do i just replace the 2 tires
2) do i use this as an opportunity to upgrade to 15" and get all new rims
and tires.

$100 compared to ????

what is the benefit if any with changing to 15"

Jeff