This series is divded into two categories: the Premier League, and the Minor League. Premier League drivers are entitled to use faster cars; Minor League drivers will race slower cars. When all the results are submitted, the fastest drivers in the Minor League will be promoted to the Premier League at the expense of the slowest drivers in the Premier League. Because you're a newcomer, you have to run in the Minor League to begin with, since the Premier League drivers for this week have already been decided.
For this week, the rally in question is Norway, with the Premier League driving Group B cars and the Minor League driving 1970s cars. Since you're a Minor League driver, you're entitled to use any car that you like in the 1970s class - the Stratos, the Escort Mk. II, the 131 Abarth or the Kadett. The choice is up to you; it largely comes down to personal preference. Even if there are difference in performance between those four cars, it doesn't really matter; rallying is a very different discipline to circuit racing, so it's entirely possible to set competitive times in slower cars. Take the Monte Carlo stages, for example - the current World Record time on Sospel for Group B cars is held my a Metro 6R4, even though it's the least-powerful car in the class.